The Moss Plant: Gametophyte
Moss plants are small, non-vascular plants belonging to the division Bryophyta. They are commonly found in moist environments such as forests, swamps, and bogs. Mosses have a unique life cycle that alternates between two generations - the gametophyte and sporophyte.

Life Cycle of Moss

1. Spore Germination: The life cycle of moss begins when a haploid spore lands on a suitable substrate. Under favorable conditions, the spore germinates and develops into a filamentous structure called the protonema.

2. Protonema: The protonema is the gametophytic stage of the moss plant. It is a thin, green, thread-like structure that grows and spreads across the substrate. The protonema produces buds, which develop into the leafy gametophores.

3. Gametophore: The gametophore is the mature gametophytic stage of the moss plant. It consists of leaf-like structures arranged in a rosette pattern. The gametophore produces specialized structures called gametangia.

4. Gametangia: The gametangia are the reproductive organs of the moss plant. They are of two types: antheridia and archegonia.

- Antheridia: The antheridia produce sperm cells (male gametes). They are flask-shaped structures that release sperm cells into the surrounding water.

- Archegonia: The archegonia produce egg cells (female gametes). They are flask-shaped structures that contain the egg cell at their base.

5. Fertilization: When water is present, the sperm cells swim to the archegonia and fertilize the egg cell. This process is known as fertilization, and it results in the formation of a diploid zygote.

6. Sporophyte Development: The zygote develops into the sporophyte, which is the diploid generation of the moss plant. The sporophyte remains attached to the gametophyte and is dependent on it for nutrients.

7. Sporangium: The sporophyte consists of a stalk-like structure called the seta, topped by a capsule called the sporangium. The sporangium produces spores through meiosis.

8. Spore Release: When the spores are mature, the sporangium bursts open, releasing the spores into the environment.

9. Spore Germination: The spores disperse and land on suitable substrates, where they can germinate and grow into new gametophytes, completing the life cycle of the moss plant.

Conclusion
In conclusion, the moss plant is predominantly a gametophyte with a sporophyte attached to it. The gametophyte stage is the dominant and independent stage, while the sporophyte stage is short-lived and dependent on the gametophyte for nutrition. Therefore, option A, "Gametophyte," is the correct answer.