[A]: One diploid embryo sac mother cell develops within the nucellus of the ovule.
[R]: Embryo sac mother cell divides meioti- cally and one of the cells produced becomes the haploid embryo sac.

Options

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

Answer

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**: The assertion states that "one diploid embryo sac mother cell develops within the nucellus of the ovule."
- This is correct because the ovule, also known as the megasporangium, contains the embryo sac mother cell, which is diploid (2n).

2. **Understanding the Reason (R)**: The reason states that "the embryo sac mother cell divides meiotically and one of the cells produced becomes the haploid embryo sac."
- This is also correct. The diploid embryo sac mother cell undergoes meiosis, resulting in the formation of four haploid cells, out of which typically three degenerate, leaving one functional haploid cell that develops into the embryo sac.

3. **Evaluating the Relationship Between A and R**:
- While both the assertion and reason are true, the reason does not serve as a correct explanation for the assertion. The assertion describes the presence and nature of the embryo sac mother cell, while the reason describes the process of its division and the formation of the embryo sac.

4. **Conclusion**: Since both statements are true but the reason does not explain the assertion, we conclude that:
- The correct answer is that both A and R are true, but R is not the correct explanation of A.

### Final Answer:
Both A and R are true, but R is not the correct explanation of A.

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