Assertion : Self-incompatibility is a genetic mechanism which prevents self-pollination and thereby fertilisation by inhibiting either pollen germination or pollen tube growth in the pistil.
Reason : In gametophytic self-incompatibility, the incompatibility reaction is determined by the genotype of the sporophytic tissue of the plant from which the pollen is derived.

Options

A. If both assertion and reason are true and reason is the correct explanation of assertion
B. If both assertion and reason are true but reason is not the correct explanation of assertion
C. If assertion is true but reason is false
D. If both assertion and reason are false

Explanation

To analyze the given assertion and reason, we will break down the concepts of self-incompatibility and the types of self-incompatibility mechanisms.

### Step-by-Step Solution:

1. **Understanding Self-Incompatibility**:
- Self-incompatibility is a genetic mechanism that prevents self-pollination and fertilization. It does this by inhibiting either the germination of pollen grains or the growth of the pollen tube within the pistil of the flower.

2. **Assertion Analysis**:
- The assertion states that self-incompatibility prevents self-pollination and fertilization by inhibiting pollen germination or pollen tube growth. This is a true statement as it accurately describes the function of self-incompatibility.

3. **Understanding Gametophytic Self-Incompatibility**:
- The reason provided states that in gametophytic self-incompatibility, the incompatibility reaction is determined by the genotype of the sporophytic tissue of the plant from which the pollen is derived. This statement is incorrect.
- In gametophytic self-incompatibility, the incompatibility is actually determined by the genotype of the pollen (male gametophyte), not the sporophytic tissue.

4. **Conclusion**:
- Since the assertion is true and the reason is false, we conclude that the correct option is that the assertion is true but the reason is false.

### Final Answer:
- Assertion: True
- Reason: False
- Therefore, the correct option is C.