Which of the following terms best describes why a skier sliding down a hill eventually comes to a stop?
a. acceleration
b. inertia
c. gravity
d. friction

Acceleration means you go faster 
inertia means it has a tendency to do nothing or remain unchanged
gravity is what pulls things down to earth
Friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. 
so i would say friction  

D. Friction because friction is the force resisting the relative motion of solid surfaces, fluid layers, and material elements sliding against each other. The friction between the two surfaces converts kinetic energy into thermal energy.