Question

A box slides down a ramp. This is the free body diagram for the box.
Which force represents friction?

A

B

C

none

Answer 1

Think about which way the box would slide on the ramp. The frictional force acts in the opposite direction of that motion.

Answer 2

Friction acts opposite to the motion. So as show in figure the box is sliding down i.e. the friction force will be "C" upwards.