The correct option is A Polytrichum
Gametophyte
is the male part of the plant. Largest gametophyte occurs in mosses. Out of
given options, only Polytrichum is a moss. So, it has largest gametophyte. It
is the predominant stage of the life cycle of a moss. The plant body is a leafy
gametophyte which has multicellular and branched rhizoids. They consist of
upright, slender axis bearing spirally arranged leaves.