Option 1 - Applied force: This refers to a force that is intentionally applied to an object. In the case of a vibrating pendulum bob, an applied force could be someone pushing or pulling the bob to set it in motion. However, this is not an example of the force of gravity of the earth acting on the bob.
Option 2 - Frictional force: Frictional force occurs when two objects rub against each other, causing resistance to motion. While friction may act on a pendulum bob, it is not related to the force of gravity acting on it.
Option 3 - Restoring force: This is the correct answer. When a pendulum bob is displaced from its equilibrium position, the force of gravity acts as a restoring force. It pulls the bob back towards the equilibrium position, causing it to oscillate.
Option 4 - Virtual force: Virtual force is not a valid term in the context of physics or the force of gravity acting on a pendulum bob. It does not accurately describe the situation.
Therefore, the correct answer is option 3, as the force of gravity of the earth acts as the restoring force on a vibrating pendulum bob, pulling it back towards its equilibrium position.