Answer
The answer is a, b, and d.
Work Step by Step
In this case the text asks you to list all the forces that apply, we will walk through each scenario.
a. gravity acting downwards. This applies as gravity is always present.
b. the normal force acting upward. This applies because the rock is skidding across the ground. The normal force is equal to the gravitational force. There is no y-component of the force.
c. the force of the kick. This does NOT apply because we know that in this case the kick is the agent which acts on the rock and not a force.
d. Friction acting opposite the direction of motion. We know that this does apply because the action did not occur on a frictionless surface and kinetic friction acts opposite the direction of motion.
Therefore the correct answer is a,b, and d.
