Choose the case where minimum friction acts
(a) When a person walks on the road.
(b) When a person walks on wet marble floor.
(c ) When a person walks on sand floor.

Explanation

To determine the case where minimum friction acts, we need to analyze the three options provided:

1. **When a person walks on the road**:
- The surface of the road is typically rough, which means there is a significant amount of friction acting between the shoes and the road. This friction helps the person to walk without slipping.

2. **When a person walks on a wet marble floor**:
- A wet marble floor is smooth and slippery. When the floor is wet, the friction between the shoes and the marble decreases significantly. This means that the person walking on a wet marble floor experiences much less friction compared to walking on a rough surface like the road.

3. **When a person walks on a sand floor**:
- Walking on sand creates a lot of friction because the sand particles can grip the shoes. This friction is generally higher than that on a wet marble floor, making it harder to walk.

After analyzing all three cases, we can conclude that:

- The **wet marble floor** (option B) has the least amount of friction. This is because it is smooth and slippery, especially when wet, allowing for minimal resistance to movement.

### Final Answer:
**Option B: When a person walks on a wet marble floor.**

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