Free-central placentation is found in

The correct option is Dianthus.

Placentation refers to the arrangement of ovules within the ovary of a flower. In free-central placentation, the ovules are borne on a central column or axis within a single-chambered (unilocular) ovary, and septa (partition walls) are completely absent.
This type of placentation is characteristic of families like Caryophyllaceae.

Let's look at the options provided:
1. Dianthus: The ovary is unilocular, and the septa are absent. The ovules are attached to a central axis that rises from the base of the ovary. Thus, it shows free-central placentation.
2. Argemone: Displays parietal placentation, where the ovules develop on the inner wall of the ovary.
3. Brassica: Displays parietal placentation (often with a false septum called replum).
4. Citrus: Displays axile placentation, where the ovary is multilocular and the ovules are attached to the central axis in the angles formed by the septa.

Therefore, free-central placentation is found in Dianthus.