Understand the life cycle of ferns, which includes both a sporophyte and a gametophyte stage. The sporophyte is the dominant stage, typically what we recognize as a fern plant.
Recognize that the gametophyte stage in ferns is a small, heart-shaped structure known as a prothallus. It is responsible for sexual reproduction and is much smaller than the sporophyte.
Identify the environmental conditions required for the fern gametophyte to thrive. It requires moist soil to ensure adequate water supply for fertilization and growth.
Consider the options given: the underside of the leaf, attached to the underground stem, in a freshwater stream, and inside a dissected seed. Evaluate which environment provides the necessary moisture and conditions for the gametophyte.
Conclude that the fern gametophyte is typically found on moist soil, where it can access the water needed for its reproductive processes.