Vascular plants, like fern, show alternation of generations. which represents a gametophyte in the life cycle of term?

Prothallus represents a gametophyte in the life cycle of fern.

A prothallium, or prothallus (from Latin pro = forwards and Greek θαλλος (thallos) = twig) is usually the gametophyte stage in the life of a fern or other pteridophyte. Occasionally the term is also used to describe the young gametophyte of a liverwort or peat moss as well.

so the answer should be prothallus