Assertion (A): The chances of having a child with Down's syndrome increases if the age of the mother is between 20 to 25.
Reason ( R) : The chances of having a child with Downs syndrome increases with the age of the mother because age adversely affects meiotic chromosome behaviour.

Options

A. Both assertion (A) and reason ( R) are true and reason ( R) is the correct explanation of assertion (A).
B. Both assertion (A) and reason ( R) are true but reason ( R) is not the correct explanation of assertion (A).
C. Assertion (A) is true but reason ( R) is false.
D. Assertion (A) is false but reason ( R) is true.

Explanation

To solve the question regarding the assertion and reason about Down's syndrome and maternal age, we can break it down into clear steps:

### Step-by-Step Solution:

1. **Understanding the Assertion (A)**:
- The assertion states that the chances of having a child with Down's syndrome increases if the age of the mother is between 20 to 25.
- **Evaluation**: Research indicates that the risk of Down's syndrome is relatively low for mothers in this age group. The risk becomes more significant as maternal age increases, particularly after age 35.

2. **Understanding the Reason (R)**:
- The reason states that the chances of having a child with Down's syndrome increases with the age of the mother because age adversely affects meiotic chromosome behavior.
- **Evaluation**: This statement is true. As women age, the likelihood of errors during meiosis, such as non-disjunction (failure of chromosomes to separate properly), increases, leading to conditions like Down's syndrome.

3. **Conclusion**:
- Since the assertion is false (the risk is not significantly higher at ages 20-25) and the reason is true (age does affect meiotic behavior), we conclude that the assertion does not correctly explain the reason.
- Therefore, the correct answer is that Assertion (A) is false, and Reason (R) is true.

### Final Answer:
- Assertion (A) is false, and Reason (R) is true.