When a rigid body is suspended vertically and it oscillates with a small amplitude under the action of the force of gravity, the body is known as

Answer: Option B

When a rigid body is suspended vertically and it oscillates with a small amplitude under the action of the force of gravity, the body is known as

Option A: Simple pendulum
This option is not correct. A simple pendulum consists of a mass (called the pendulum bob) attached to a string or rod and swings back and forth under gravity. It is not a rigid body.

Option B: Compound pendulum
This is the correct answer. When a rigid body, such as a physical rod or a physical object, is suspended vertically and allowed to oscillate under gravity, it is referred to as a compound pendulum. The oscillation of the rigid body is similar in nature to that of a simple pendulum.

Option C: Torsional pendulum
A torsional pendulum is different from a compound pendulum. It consists of a rod or wire that is twisted and oscillates in torsional motion.

Option D: Second's pendulum
A Second's pendulum is a specific type of simple pendulum with a length such that it completes one full oscillation in exactly two seconds. It is not related to a rigid body.

Therefore, the correct answer is Option B: Compound pendulum.