Assertion : In a DNA molecule, A-T rich parts melt before G-C rich parts.
Reason : In between A and T there are three H-bond, whereas in between G and C there are two H-bonds

Options

A. Assertion is correct, reason is correct, reason is a correct explanation for assertion.
B. Assertion is correct, reason is correct, reason is not a correct explanation for assertion
C. Assertion is correct, reason is incorrect
D. Assertion is incorrect, reason is correct.

Explanation

To solve the question, we need to analyze both the assertion and the reason provided.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that in a DNA molecule, A-T rich parts melt before G-C rich parts.
- This is based on the concept of DNA melting temperature, which is influenced by the number of hydrogen bonds between base pairs.

2. **Analyzing the Base Pairing**:
- Adenine (A) pairs with Thymine (T) through **2 hydrogen bonds**.
- Guanine (G) pairs with Cytosine (C) through **3 hydrogen bonds**.
- Therefore, A-T pairs are held together by fewer hydrogen bonds compared to G-C pairs.

3. **Conclusion from the Assertion**:
- Since A-T pairs have fewer hydrogen bonds, they require less energy to separate (or "melt") compared to G-C pairs.
- Thus, the assertion that A-T rich parts melt before G-C rich parts is **correct**.

4. **Understanding the Reason**:
- The reason states that in between A and T there are **three** hydrogen bonds, whereas in between G and C there are **two** hydrogen bonds.
- This is incorrect because, as established, A-T pairs have **two** hydrogen bonds, and G-C pairs have **three** hydrogen bonds.

5. **Conclusion from the Reason**:
- Since the reason provided is incorrect, it does not support the assertion.

6. **Final Answer**:
- The assertion is correct, but the reason is incorrect. Therefore, the correct choice is: **Assertion is correct but Reason is incorrect**.