The correct answer is - Gametophyte.
Mosses are bryophytes whose life cycle alternates with gametophyte and sporophyte. A gametophyte is haploid and forms the main plant body which is the dominant phase of moss plant. On gametophyte, male and female sex organs develop which produces male and female gametes respectively. After fusion of gametes zygote is formed which develops into sporophyte. The sporophyte is attached and dependent on the gametophyte for the nutrition.