When a metal is heated, the free electrons gain
c. kinetic energy
Explanation:
Kinetic energy is an energy possessed  by an object  due to its movement.
Metals contain free electrons  which are called delocalised electrons, they move freely, when a metal is heated they vibrate and  move more quickly - this electrons gain kinetic energy from the excess vibrating ions. They move around thereby transferring some of their energy to the nearby particles