The correct answer is A. Simple pendulum.
A simple pendulum is a weight suspended from a pivot so that it can swing back and forth under the influence of gravity. The time for one complete swing, called the period, depends only on the length of the pendulum and the acceleration due to gravity.
A compound pendulum is a rigid body that swings about a fixed point. The period of a compound pendulum depends on its mass, shape, and the point about which it is swinging.
A second’s pendulum is a simple pendulum that has a period of exactly 2 seconds. The length of a second’s pendulum is 0.994 m (3 feet 3 inches) at sea level and at a temperature of 0 degrees Celsius.
A simple pendulum is a good approximation of a compound pendulum when the mass of the pendulum is concentrated at a point and the pivot is close to the center of mass of the pendulum.