In every substance, the constituent particles are moving. The magnitude of this movement is partially dependent on the state of the substance and also dependent on the amount of energy the substance possesses. As the energy contained by the particles increases, their movement is enhanced; it becomes more rapid and its magnitude increases. The energy due to motion is known as kinetic energy. Therefore, the thermal energy of an object is the energy due to the motion of the particles, or the particles' kinetic energy. Thus, the answer is C.
