A prothallus is

Options

A. a structure in pteridophytes formed before the thallus develops
B. a sporophytic free living structure formed in pteridophytes
C. a gametophyte free living structure formed in pteridophytes
D. a primitive structure formed after fertilisation in pteridophytes.

Explanation

**Step-by-Step Solution:**

1. **Understanding Prothallus**: A prothallus is a structure found in the plant kingdom, specifically in the phylum of pteridophytes.

2. **Origin of Prothallus**: The prothallus originates from spores produced by the sporophyte generation of pteridophytes. The sporophyte is the dominant phase in the life cycle of pteridophytes and is diploid (2N).

3. **Sporangia and Spores**: Within the sporophyte, there are leaf-like structures called sporophylls, which contain sporangia. Inside these sporangia, spore mother cells undergo meiosis to produce haploid (N) spores.

4. **Germination of Spores**: When these spores germinate, they develop into a small, multicellular structure known as the prothallus.

5. **Characteristics of Prothallus**: The prothallus is free-living, meaning it can exist independently from the parent plant. It is primarily photosynthetic, allowing it to produce its own food. The body of the prothallus is thalloid in structure, which means it does not have true roots, stems, or leaves.

6. **Gametophyte Stage**: Importantly, the prothallus represents the gametophyte stage of the plant life cycle, which is haploid (N). This is in contrast to the sporophyte stage, which is diploid (2N).

7. **Conclusion**: Therefore, a prothallus is defined as a gametophytic, free-living structure formed in pteridophytes.

**Final Answer**: A prothallus is a gametophytic, free-living structure formed in pteridophytes.

---