Assertion : Rate of reaction
`CD_(4)+Cl_(2)rarr" Slow "CH_(4)+Cl_(2)rarr"Fast."`
Reason : Due to isotopic effect.

Options

A. If both Assertion & Reason are True & the Reason is a correct explanation of the Assertion.
B. If both Assertion & Reason are True but Reason is not a correct explanation of the Assertion.
C. If Assertion is True but the Reason is False.
D. If both Assertion & Reason are False.

Answer

### Step-by-Step Solution:

1. **Understanding the Reactions**:
- The two reactions given are:
- \( \text{CD}_4 + \text{Cl}_2 \rightarrow \text{Slow} \)
- \( \text{CH}_4 + \text{Cl}_2 \rightarrow \text{Fast} \)

2. **Identifying the Assertion**:
- The assertion states that the reaction of \( \text{CD}_4 \) with \( \text{Cl}_2 \) is slow, while the reaction of \( \text{CH}_4 \) with \( \text{Cl}_2 \) is fast.

3. **Identifying the Reason**:
- The reason provided is that this difference in reaction rates is due to the isotopic effect.

4. **Explaining the Isotopic Effect**:
- The isotopic effect refers to the change in reaction rates due to the presence of isotopes. In this case, deuterium (\( \text{D} \)) is heavier than hydrogen (\( \text{H} \)).
- The bond between carbon and deuterium (\( \text{C-D} \)) is stronger and less vibrationally active compared to the bond between carbon and hydrogen (\( \text{C-H} \)).

5. **Analyzing the Strength of Bonds**:
- The \( \text{C-D} \) bond is stronger than the \( \text{C-H} \) bond because deuterium is heavier, leading to less vibrational motion.
- As a result, \( \text{CD}_4 \) is more stable and reacts slower with \( \text{Cl}_2 \).

6. **Comparing Reaction Rates**:
- In contrast, \( \text{CH}_4 \) has a weaker \( \text{C-H} \) bond, which vibrates more due to the lighter mass of hydrogen. This increased vibrational energy makes \( \text{CH}_4 \) less stable and more reactive, leading to a faster reaction with \( \text{Cl}_2 \).

7. **Conclusion**:
- Both the assertion and the reason are true, and the reason correctly explains the assertion. Therefore, the correct answer is that both the assertion and reason are true, and the reason is a correct explanation of the assertion.

### Final Answer:
Both the assertion and the reason are true, and the reason is a correct explanation of the assertion.