The phenomenon in which the amplitude of oscillation of a pendulum decreases gradually is called
A. Decay period of oscillation
B. Damping
C. Building up of oscillation
D. Maintained oscillation

Hint: To find the correct option as provided in the given question we have to know the motion of the object when force is applied on it. In a simple harmonic motion (SHM) an object moves to and fro motion about their mean position.
A simple pendulum contains a small mass which is connected by a string to a fixed point. When a force is applied to the pendulum then the pendulum swings to and fro with periodic motion.
The period of the pendulum does not depend on the mass of the object. It depends on the length of the string

Complete step by step answer:
The decay period of oscillation is defined as the total time taken to stop the motion when force acts on an object. The decay time of the oscillation depends on the length of the string and the force applied to the pendulum.
In physics, a damping is defined as the process in which the amplitude of oscillation of a pendulum decreases when force is applied. The damping restricts or prevents the oscillation of the system that reduces the oscillation gradually.
The building up oscillation is defined as an oscillation that builds up with a much longer time constant or dies down with much a longer time constant than the fundamental frequency of the oscillation. In build up oscillation the amplitude of the oscillation increases through an external force.
The maintained oscillation is defined as the process in which the amplitude of the oscillation remains constant by applying external force. If the continuous force is applied on the system like a pendulum, then the oscillation of the system remains constant.
Therefore, the phenomenon in which the amplitude of oscillation of a pendulum decreases gradually is called as damping

So, the correct answer is “Option B”.

Note:
In a damping phenomenon, the amplitude of the oscillation is decreasing gradually instead of sudden decreasing. When there is a damping in the system then the amplitude decreases and the period increases.