What type of energy is defined as the capacity to do work?

The appropriate choice refers to mechanical energy, which is defined as the sum of kinetic and potential energy in a system. Mechanical energy is indeed the capacity to do work, as it encompasses both the energy of motion (kinetic) and the energy stored due to an object's position (potential).

Kinetic energy is specifically the energy possessed by an object due to its motion, while potential energy is the energy stored in an object because of its position or state, such as an object held at a height above the ground. Thermal energy, on the other hand, is related to the internal energy of a system due to the random motion of particles and does not directly relate to the capacity to perform work in the classical mechanical sense.

By recognizing that mechanical energy accounts for both kinetic and potential energy, it is clear why it is defined as the capacity to do work, as it can be transformed into other forms of energy to perform tasks, such as moving objects or generating heat.