Because It is a combination of kinetic energy and potential energy. Kinetic energy is the one that is due to the motion of an object whereas potential energy due to the virtue of an objects position.

Explanation:

The energy possessed by an oscillating pendulum of a clock is mechanical energy. Mechanical energy is the sum of kinetic energy and potential energy in a system.

1. Kinetic Energy:
Kinetic energy is the energy of motion. In the case of a pendulum, when the bob is moving from one side to another, it possesses kinetic energy. At the highest point of its swing, the bob momentarily comes to rest before reversing its direction. At this point, the kinetic energy is zero.

2. Potential Energy:
Potential energy is the energy possessed by an object due to its position or state. In the case of a pendulum, when the bob is at the highest point of its swing, it possesses maximum potential energy. As the bob swings down, its potential energy decreases and is converted into kinetic energy. At the lowest point of its swing, the bob possesses minimum potential energy and maximum kinetic energy.

3. Restoring Energy:
Restoring energy is not a recognized form of energy. It seems to be a misnomer in this context.

4. Mechanical Energy:
Mechanical energy is the sum of kinetic energy and potential energy in a system. In the case of a pendulum, as it oscillates back and forth, there is a continuous interconversion of kinetic and potential energy. At any given point in time, the total mechanical energy remains constant. For example, when the pendulum is at the highest point of its swing, it has maximum potential energy and zero kinetic energy. As it swings down, potential energy decreases and kinetic energy increases, but the total mechanical energy remains constant.

Therefore, the correct answer is option D) mechanical energy.