Assertion: Friction opposes relative motion and thereby dissipates power in the from of heat.
Reason: Friction is always an undesirable force.

Options

A. Both assertion and reason are true and reason is the correct explanation of assertion.
B. Both assertion and reason are true and reason is the not correct explanation of assertion.
C. The assertion is true but reason is false
D. Both assertion and reason are false.

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 "Friction opposes relative motion and thereby dissipates power in the form of heat."
- This is true because friction acts in the opposite direction to the motion of an object. When two surfaces are in contact and one moves relative to the other, friction does work against that motion. This work done against friction converts kinetic energy into thermal energy (heat), leading to the dissipation of power.

2. **Understanding the Reason**:
- The reason states that "Friction is always an undesirable force."
- This statement is not entirely accurate. While friction can be undesirable in some situations (like when it causes wear and tear or requires more energy to move an object), it is also essential in many scenarios. For instance, friction is necessary for walking, driving, and holding objects. Without friction, we would not be able to perform these everyday activities effectively.

3. **Conclusion**:
- Based on the analysis, the assertion is true because it correctly describes the nature of friction and its effects. However, the reason is false because it incorrectly categorizes friction as always undesirable.
- Therefore, the correct answer is that the assertion is true, but the reason is false.

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