Internal energy changes that do not register on a thermometer are due to changes in the energy of the particles.

The correct answer is potential energy.

Explanation:

Internal energy is defined as the energy associated with the disordered motion of the molecules. The potential energy is associated with the inter molecules attractive forces in liquid and solid.

In polyatomic gases, there is vibrational kinetic energy and the translational kinetic energy.

Temperature is the measure of average kinetic energy.

Therefore, internal energy changes that do not register on a thermometer are due to changes in the potential energy of the particles.