Which one of the following is an example of the force of gravity of the earth acting on a vibrating pendulum bob?

The correct answer is C, Restoring force. For a swinging pendulum, the component of the bob's weight along its path always pulls it back towards the lowest point, so gravity acts as a restoring force. When the bob is pulled to one side, its weight mg can be split into two parts: mg cos θ along the string, balanced by the tension, and mg sin θ along the arc, directed towards the mean position. This second part is the restoring force. For small swings it is roughly proportional to the displacement, so the bob performs simple harmonic motion with a time period of 2π times the square root of length divided by g, which does not depend on the mass of the bob. Option A is wrong because an applied force is an outside push or pull, such as the hand that starts the swing. Option B is wrong because friction and air resistance oppose motion and slowly damp the swing. Option D is wrong because virtual force is not the force that brings the bob back. Exam tip: pendulum restoring force is mg sin θ; period depends on length and g, not mass.