Why does a pendulum have periodic motion?

A) the pendulum has a spring constant

B) the pendulum obeys Hooke's Law

C) the spring pulls on the pendulum

D) gravity acts on the pendulum

A, B, and C are all incorrect because they are talking about springs. I'm assuming you're talking about a 2 dimensional, side to side pendulum. In that case, the answer is D because as the pendulum goes up from one side, gravity accelerates it downward until it swings to the other side. If you're not talking about this type of pendulum, leave a comment and ill come back.