Placentation

Reference: NCERT Class XI, page 65.

When the placenta is axial and the ovules are attached to it in a multilocular ovary, the placentation is axile.
It is seen in tomato, lemon and china rose.

$Dianthus$ (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 $Dianthus$.

In basal placentation the placenta develops at the base of the ovary and a single ovule is attached to it; the ovary is unilocular. Examples: sunflower and marigold.
In axile placentation the ovules are attached to a central axis in a multilocular ovary (china rose, tomato, lemon).
In marginal placentation the ovules are borne in rows on a ridge along the ventral suture (pea).
In free central placentation many ovules are borne on a central axis without septa ($Dianthus$, primrose).
Hence a unilocular ovary with a single ovule has basal placentation.

In axile placentation the placenta is axial and the ovules are attached to it in a multilocular ovary, as in china rose, tomato and lemon.
Marigold (and sunflower) have basal placentation: a single ovule attached at the base of a unilocular ovary.
$Argemone$ (and mustard) have parietal placentation: ovules on the inner wall of the ovary.
$Dianthus$ (and primrose) have free central placentation: ovules on a central axis without septa.
Hence the example of axile placentation is lemon.

Placentation is the arrangement of ovules within the ovary.
Sweet pea (Lathyrus odoratus) belongs to the family Fabaceae (Leguminosae).
Its gynoecium is monocarpellary with a superior, unilocular ovary.
The placenta forms a ridge along the ventral suture of the ovary, and the ovules are borne on this ridge in two rows. This is marginal placentation.
Basal placentation is seen in sunflower and marigold.
Axile placentation is seen in china rose, tomato and lemon.
Free central placentation is seen in Dianthus and Primrose.
So the placentation in sweet pea is marginal.