Assertion :- In a redox reaction, the oxidation number of the oxidant decreases while that of reductant increases.
Reason :- Oxidant gains electron(s) while reductant loses electron(s).

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

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

### Step-by-Step Solution:

1. **Understanding Redox Reactions**:
- A redox (reduction-oxidation) reaction involves the transfer of electrons between two species. In this process, one species is oxidized (loses electrons) and the other is reduced (gains electrons).

2. **Assertion Analysis**:
- The assertion states: "In a redox reaction, the oxidation number of the oxidant decreases while that of the reductant increases."
- In a redox reaction, the oxidant (the species that gains electrons) indeed has its oxidation number decrease. Conversely, the reductant (the species that loses electrons) has its oxidation number increase. Thus, the assertion is correct.

3. **Reason Analysis**:
- The reason states: "Oxidant gains electron(s) while reductant loses electron(s)."
- This statement accurately describes the behavior of oxidants and reductants in a redox reaction. The oxidant gains electrons, leading to a decrease in its oxidation state, while the reductant loses electrons, resulting in an increase in its oxidation state. Therefore, the reason is also correct.

4. **Conclusion**:
- Since both the assertion and the reason are correct, and the reason correctly explains the assertion, we conclude that the answer is option (a) - both the assertion and reason are correct, and the reason is the correct explanation of the assertion.

### Final Answer:
The answer for the given question is option (a).

---