Assertion: Mass of a body decreases slightly when it is negatively charged.
Reason: Charging is due to transfer of electrons.

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 the mass of a body decreases slightly when it is negatively charged.
- To evaluate this, we need to consider what happens when a body becomes negatively charged.

2. **Charging a Body Negatively**:
- A body becomes negatively charged when it gains electrons. Electrons have a mass of approximately \(9.1 \times 10^{-31}\) kg.
- Therefore, when electrons are added to a body, the total mass of the body should actually increase slightly, not decrease.

3. **Conclusion on the Assertion**:
- Since the assertion claims that the mass decreases when it is negatively charged, and we have established that the mass actually increases due to the addition of electrons, the assertion is **incorrect**.

4. **Understanding the Reason**:
- The reason states that charging is due to the transfer of electrons.
- This is a correct statement because charging can occur through the addition (gaining electrons) or removal (losing electrons) of electrons.

5. **Conclusion on the Reason**:
- The reason is indeed correct as it accurately describes the process of charging a body.

6. **Final Conclusion**:
- The assertion is wrong, while the reason is correct. Thus, the overall conclusion is that the assertion is false and the reason is true.

### Summary of the Solution:
- **Assertion**: Incorrect (mass increases when negatively charged).
- **Reason**: Correct (charging involves transfer of electrons).