Placentation refers to the arrangement or mode of attachment of ovules within the ovary. The placenta is the tissue inside the ovary wall to which ovules are attached. It provides nutrition to the developing ovules through the funicle (stalk connecting ovule to placenta). The position and arrangement of placenta varies enormously among angiosperms and is used as a taxonomic character. The type of placentation is related to the number of carpels, whether the carpels are free (apocarpous) or fused (syncarpous), and the number of locules (chambers) in the ovary. Understanding placentation is important for identifying plant families and genera in systematic botany. The different types of placentation are named for their anatomical position within the ovary.

In marginal placentation, the placenta forms a ridge along the ventral suture (inner side) of the ovary, and ovules are attached to this ridge. The ovary has a single chamber (unilocular) and a single carpel (monocarpellary). The placenta runs along the margin (ventral suture) of the single folded carpel. Classic example: Pea (Pisum sativum). Family: Fabaceae (Leguminosae). Also: beans (Phaseolus), gram (Cicer), groundnut (Arachis). When pea pods are opened along their "seam" (dorsal suture), you can see the peas attached to the opposite edge (ventral suture) — this is marginal placentation in action. The pea pod is a monocarpellary, superior ovary that develops into the legume fruit. This type of placentation is characteristic of the family Fabaceae, one of the most economically important plant families.

In axile placentation, the ovary is multilocular (2 or more chambers) formed by the fusion of multiple carpels (syncarpous). The placentae are located on the central axis (axile), where the septa (partitions dividing the ovary into locules) meet at the centre. Ovules are attached to the central axis. Classic examples: Lemon (Citrus limon) — multicarpellary syncarpous, many locules, each locule with 1-2 ovules attached to central axis. Also: Tomato (Lycopersicon esculentum), China rose/Hibiscus (Hibiscus rosa-sinensis), Onion (Allium cepa), Tulip (Tulipa). The distinct wedge-shaped locules of a cut lemon or orange, with seeds attached to the central pillar, clearly illustrate axile placentation. Tomato: 2-many carpels fused → bilocular or multilocular ovary with seeds on central axis.

In parietal placentation, the ovary is unilocular (single chamber) formed by two or more fused carpels (syncarpous), but the carpel walls remain separate in the interior — no septa form. The placentae develop on the inner wall (parietal = wall) of the ovary or on incomplete septa projecting inward. The ovary appears as a single large chamber with ovules attached to the sides. Classic example: Mustard (Brassica campestris/Brassica juncea) — bicarpellary, syncarpous, superior ovary with parietal placentation. Initially appears unilocular but develops a false septum (replum) — making it appear bilocular (this is why mustard fruits are bilocular despite originally being unilocular with parietal placentation). Also: Poppy (Papaver), Argemone, Cucumber family (Cucurbitaceae — watermelon, pumpkin). Marigold (Tagetes) — discussed below.

In basal placentation, a single ovule is attached to the base of the unilocular ovary. The placenta is located at the bottom (base) of the ovary, bearing typically just one ovule. Classic example: Marigold (Tagetes) and other members of Asteraceae (sunflower family). Also: Wheat (Triticum), Rice (Oryza), Sunflower (Helianthus annuus). In Asteraceae, the ovary is inferior, unilocular, with a single basal ovule. The seeds of marigold, sunflower, wheat, and rice develop from this single basally-attached ovule. This is why each floret of a sunflower produces only one seed, and each wheat grain is technically a single seed in a unilocular ovary. Basal placentation is characteristic of the Asteraceae and Poaceae families.

In free central placentation, the ovary is unilocular (no septa) and the ovules are attached to a central column of tissue that arises from the base of the ovary but is NOT connected to the ovary wall. This central axis floats freely in the single chamber. Classic example: Dianthus (carnation/sweet william), Primrose (Primula). Also: Stellaria (chickweed). Family: Caryophyllaceae. The central column in Dianthus bears many ovules on its surface. This type of placentation is derived from axile placentation by dissolution of the septa — the central axis remains but the locule divisions disappear.

In superficial (laminar) placentation, the placentae develop on the inner surface of the ovary wall across the entire surface of the septa/partitions in a multilocular ovary. Ovules appear to cover the inner surface of each locule. This is found in some aquatic plants. Classic example: Water lily (Nymphaea). Also: Nelumbo (lotus) — though lotus has separate carpels (apocarpous), each with a single ovule. Superficial placentation is relatively rare and mostly found in aquatic angiosperms. The inner wall of each locule is covered with ovules — giving a 'wallpapered' appearance to the cut ovary.

Marginal: monocarpellary, unilocular, placenta along ventral suture. Example: Pea. Axile: multicarpellary, syncarpous, multilocular, placenta at central axis where septa meet. Examples: Lemon, Tomato, China rose, Onion. Parietal: bi/multicarpellary, syncarpous, unilocular (no septa or incomplete septa), placenta on wall. Examples: Mustard, Poppy, Marigold* (though marigold has basal — see below), Argemone. Basal: monocarpellary, unilocular, single ovule at base. Examples: Marigold, Wheat, Sunflower. Free central: multicarpellary, syncarpous, unilocular, placenta on free central column. Examples: Dianthus, Primula. Superficial: multicarpellary, syncarpous, multilocular, ovules cover inner septum surface. Example: Nymphaea. Note: The NEET answer assigns Marigold (Tagetes) to BASAL placentation (I) and Mustard to PARIETAL (C→I in the match). The answer A-II, B-IV, C-I, D-III means: Marginal=Pea, Axile=Lemon, Parietal=Mustard, Basal=Marigold.