If a substance absorbs energy but its temperature does not change, then the energy is changing.
a. kinetic
b. potential

If a substance absorbs energy but its temperature does not change, then the internal POTENTIAL energy is changing.

Explanation:

As we know that the internal energy of the molecules in terms of their kinetic energy is directly related to the temperature of the gas

Here we know that

so if the energy is transferred in the form of kinetic energy of the molecules then it will increase the temperature of the gas

so here it says that the substance absorbs the energy but its temperature does not change then it means the kinetic energy of the gas will remain constant

So this energy will convert into its potential energy

so correct answer will be

If a substance absorbs energy but its temperature does not change, then the internal POTENTIAL energy is changing.