Assertion :- At constant pressure for the change `H_(2)O(s)rarr H_(2)O(g)` work done is negative.
Reason :- During phase transition work done is always negative.

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 analyze the given assertion and reason, we will break down the concepts involved in the phase transition of water from solid (ice) to gas (water vapor) and the associated work done.

### Step-by-Step Solution:

1. **Understanding the Assertion**:
- The assertion states that at constant pressure for the change \( H_2O(s) \rightarrow H_2O(g) \), the work done is negative.
- When ice (solid) converts to water vapor (gas), it undergoes a phase transition where the volume increases significantly because gases occupy more volume than solids.

2. **Work Done in Thermodynamics**:
- The work done by a system at constant pressure can be expressed as:
\[ W = -P \Delta V \]
- Here, \( \Delta V \) is the change in volume. If the volume increases (\( \Delta V > 0 \)), then the work done by the system is negative, indicating that the system is doing work on the surroundings.

3. **Volume Change During Phase Transition**:
- In the transition from solid to gas, the volume increases. Therefore, \( \Delta V \) is positive, leading to negative work done by the system.
- This confirms that the assertion is correct: the work done during the transition from ice to water vapor at constant pressure is indeed negative.

4. **Understanding the Reason**:
- The reason states that during phase transitions, the work done is always negative.
- This statement is not universally true. For example, during the transition from ice to liquid water at 0°C, the volume decreases because water has a higher density in its liquid state compared to its solid state at that temperature. In this case, \( \Delta V < 0 \), leading to positive work done on the system.

5. **Conclusion**:
- The assertion is true, but the reason is false. Therefore, the correct answer is that the assertion is true, but the reason is not.

### Final Answer:
- The assertion is true, while the reason is false.