**Friction opposing the box**

Friction is a force that opposes the motion of objects when they are in contact with each other. It acts in the opposite direction to the applied force, making it more difficult for objects to move. In this case, when loading boxes onto a truck using a ramp, friction plays a crucial role in making the process smoother.

**The role of friction in loading boxes on a truck ramp:**

1. **Preventing the box from slipping:** When a box is placed on a ramp, gravity tries to pull it downward. Without friction, the box would simply slide down the ramp, making it difficult to load onto the truck. However, friction between the box and the ramp's surface provides the necessary grip to prevent the box from slipping.

2. **Helping to control the speed of the box:** Friction also helps to control the speed at which the box moves down the ramp. If there was no friction, the box would slide down the ramp very quickly, potentially causing it to collide with other boxes or objects on the truck. Friction slows down the box's motion, allowing for better control during the loading process.

3. **Reducing the force required to push the box:** When friction is present, the force required to push the box up the ramp is reduced. Friction acts in the opposite direction to the applied force, making it easier to move the box. Without friction, the person loading the boxes would have to exert a much greater force to overcome the lack of grip between the box and the ramp.

4. **Preventing damage to the box:** Friction helps to keep the box stable and prevents it from sliding or toppling over during the loading process. This helps to minimize the risk of damage to the box and its contents.

In conclusion, friction is essential in making the loading of boxes onto a truck ramp smoother. It prevents the box from slipping, helps to control its speed, reduces the force required to push the box, and prevents damage to the box. Therefore, the correct answer is option 'C' - friction opposing the box.