Which of the following is an example of energy changing from one form to another?

a rock

a fire

a desk

a pen

Fire is an example of energy changing one form to another. It is used as thermal energy (for heating and warmth) and for burning energy.

The other options don't really have any energy.

The correct option is

a fire

In fact, a fire is a conversion of chemical energy (contained in the molecules of the initial substances) into thermal energy (the heat released by the fire itself), therefore this is an example of energy changing from one form to another. All the other objects, instead, do not represent any form of energy conversion.