Assertion: When ice melts, work is done by atmosphere on (ice+water) system.
Reason: On melting of ice volume of (ice+water) system decreases.

Options

A. (a) If both Assertion and Reason are true and the Reason is correct explanation of the Assertion.
B. (b) If both Assertion and Reason are true but Reason is not the correct explanation of Assertion.
C. (c) If Assertion is true, but the Reason is false.
D. (d) If Assertion is false but the Reason is true.

Answer

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 when ice melts, work is done by the atmosphere on the (ice + water) system.

2. **Density Consideration**:
We know that the density of ice is less than the density of water. This means that when ice melts, it transforms into water, which has a higher density.

3. **Volume Change**:
Since the density of water is greater than that of ice, the volume of the (ice + water) system decreases when ice melts. This is because the same mass of water occupies less volume than the same mass of ice.

4. **Work Done by the System**:
The work done by a system can be expressed as:
\[ W = P \Delta V \]
where \( P \) is the pressure and \( \Delta V \) is the change in volume. In this case, since the volume is decreasing (\( \Delta V < 0 \)), the work done by the system is negative.

5. **Interpreting Negative Work**:
A negative work done by the system implies that work is being done on the system by the surroundings (in this case, the atmosphere). Therefore, the atmosphere does work on the (ice + water) system as the ice melts.

6. **Understanding the Reason**:
The reason states that on melting of ice, the volume of the (ice + water) system decreases. This is indeed true as we have established that the density of water is greater than that of ice.

7. **Conclusion**:
Both the assertion and the reason are correct. Furthermore, the reason correctly explains the assertion because the decrease in volume leads to work being done on the system by the atmosphere.

### Final Answer:
Both the assertion and the reason are true, and the reason is the correct explanation of the assertion. Therefore, the correct option is:
- Option A: Both assertion and reason are true, and reason is the correct explanation of the assertion.