Prothallus represents

The correct option is "gametophytic phase in a fern".

To understand why this is correct, let us break down the life cycle and characteristics of ferns (pteridophytes):

1. Alternation of Generations:
Like other plants, ferns exhibit an alternation of generations between a multicellular diploid sporophyte phase and a multicellular haploid gametophyte phase.

2. The Prothallus:
In ferns, when a haploid spore germinates, it grows into a inconspicuous, small, green, heart-shaped, multicellular, free-living, and photosynthetic structure. This structure is called the prothallus.

3. Role of the Prothallus:
Because the prothallus develops directly from a haploid spore, all its cells are haploid (having 1n chromosomes). It bears the male sex organs (antheridia) and female sex organs (archegonia) which produce gametes. Therefore, it represents the gametophytic phase of the fern's life cycle.