Thermal energy is the only form of energy that can be added to or removed from a substance. How is thermal energy that is added to a substance stored?

Correct Answer: Option D — In the form of internal energy.

Thermal energy added to a substance is stored as internal energy, which is the sum of the kinetic and potential energies of the molecules within the substance. In thermodynamics, this internal energy is a state function that changes as heat is added or work is performed on the system.