The correct answer is option-1: Pteridophyta. The term "prothallus" refers to the gametophyte stage in the life cycle of pteridophytes. Pteridophyta is a division of plants that includes ferns, horsetails, and clubmosses. In the life cycle of pteridophytes, the sporophyte is the dominant phase, while the gametophyte is small and inconspicuous. The gametophyte, called a prothallus, develops from a spore and produces gametes (sperm and eggs). Fertilization occurs when sperm swim through a film of water to reach the egg on the prothallus. Hence, it can be concluded that the gametophyte in pteridophytes is called a prothallus.