The shape of anther is:

The correct answer is Option (2) → Tetragonal

A typical anther is bilobed: it has two lobes, and each lobe has two chambers called theca. This gives four microsporangia, one at each corner, two in each lobe. In a transverse section the anther therefore looks four-sided, which is called tetragonal.

(1) Hexagonal — Incorrect. A hexagon has six sides. The anther has four microsporangia.

(2) Tetragonal — Correct. This is the four-sided shape of the anther.

(3) Pyramidal — Incorrect. This is not the shape of an anther.

(4) Trigonal — Incorrect. A trigonal shape has three sides. The anther has four.

NCERT Extract:
Stamen, Microsporangium and Pollen Grain – “… A typical angiosperm anther is bilobed with each lobe having two theca, i.e., they are dithecous (Figure 1.2b). Often a longitudinal groove runs lengthwise separating the theca. Let us understand the various types of tissues and their organisation in the transverse section of an anther (Figure 1.3a). The bilobed nature of an anther is very distinct in the transverse section of the anther. The anther is a four-sided (tetragonal) structure consisting of four microsporangia located at the corners, two in each lobe.”