Morphology of Flowering Plants

Homologous organs have the same origin (same basic structure) although their functions may differ.
Sweet potato is a modified root (a swollen adventitious root that stores food).
Turnip is also a modified root (a swollen tap root that stores food), so the two are homologous.
Ginger (rhizome), potato (stem tuber) and $Colocasia$ (corm) are modified underground stems; they are only analogous to sweet potato.
Hence sweet potato is homologous to turnip.

In whorled phyllotaxy more than two leaves arise at a node and form a whorl.
$Alstonia$ has whorled, simple leaves with reticulate venation (it is a dicot).
China rose has alternate phyllotaxy (a single leaf at each node), as in mustard and sunflower.
$Calotropis$ has opposite phyllotaxy (a pair of leaves at each node), as in guava.
Neem has pinnately compound leaves, not simple leaves.
Hence whorled simple leaves with reticulate venation are present in $Alstonia$.

$Dianthus$ (and $Primrose$) shows free central placentation.
In free central placentation the ovules are borne on a central axis and septa are absent, so the ovary is unilocular.
The correct diagram is therefore the transverse section showing a single chamber with the ovules attached all round a central column that is not connected to the ovary wall by partitions.
The other diagrams show marginal placentation (ovules in rows along the ventral suture of a pod, as in pea), parietal placentation (ovules on the inner wall of the ovary, as in mustard) and axile placentation (ovules on a central axis in a multilocular ovary with septa, as in china rose and tomato).
Hence the diagram of free central placentation represents $Dianthus$.