The correct answer is D) In the form of internal energy. When thermal energy is added to a substance, it is stored in the form of internal energy, which is the total kinetic and potential energy of the atoms or molecules within the substance. As the thermal energy is added, it increases the kinetic energy of the particles, causing them to move faster and vibrate more, which in turn increases the internal energy of the substance. The other answer choices are incorrect because: A) Potential kinetic energy is not a recognized form of energy; B) Heat is the transfer of thermal energy, not the storage of it; and C) Mechanical energy is a distinct form of energy, separate from internal energy.