Answer: Option (a) is the correct answer.
Step-by-step explanation:
Energy obtained by an object due to the motion of particles that are present within the object is known as thermal energy.
So, when we increase the temperature of a substance then its molecules tend to gain kinetic energy. Because of this molecules will start to move from their initial position and hence, there will be more number of collisions between the molecules.
As a result, there will be increase in thermal energy of the substance.
Thus, we can conclude that in order to increase thermal energy, the temperature must increase.