Explanation
In pteridophytes, the gametophytic generation is commonly called the prothallus. The prothallus is a small, flat, heart-shaped, or kidney-shaped structure that is typically 1-2 cm in size. It develops from the spore and produces the male and female gametes that fuse to form the zygote, which develops into the sporophytic generation. Therefore, the answer is C) Prothallus.
