In addition to gravity, what is the other predominant force that affects the motion of a pendulum?

A. the force of friction

B. the force of tension

C. the force of air resistance

D. the restoring force

E. the normal force

Since it doesn't specify that it's in a vacuum there should be air resistance. So the answers C. Hope I'm right

B. the force of tension.

C. Force of air resistance.

D. restoring force

Explanation: When a pendulum swings in the presence of air, the force of air resistance applied on it and try to reduce the pendulum's speed. Force of tension also applied on the string of pendulum and affect its motion. The restoring force always tries to stabilize the pendulum and affect the motion of pendulum adversely.