The small heart-shaped gametophyte called a prothallus is part of the life cycle of the :

Options

A. mosses
B. ferns
C. liverworts
D. gnetophytes

Explanation

### Step-by-Step Solution:

1. **Identify the Structure**: The question mentions a "small heart-shaped gametophyte called a prothallus." This is a key term that relates to the reproductive phase of certain plants.

2. **Understand the Role of Prothallus**: The prothallus is the gametophyte stage in the life cycle of ferns. It is responsible for producing gametes (male and female).

3. **Life Cycle Context**: In the life cycle of ferns (which belong to the group called pteridophytes), the prothallus develops from a spore. This is an important transition from the sporophyte generation to the gametophyte generation.

4. **Fertilization and Development**: After fertilization occurs between the male and female gametes produced by the prothallus, a new sporophyte plant begins to develop. This sporophyte will consist of a primary root, a primary leaf, and the rudiment of a new stem.

5. **Conclusion**: Therefore, the small heart-shaped gametophyte called a prothallus is part of the life cycle of ferns, which are classified under pteridophytes.

### Final Answer:
The small heart-shaped gametophyte called a prothallus is part of the life cycle of ferns (pteridophytes).

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