Option B, because the metal thermally expands and contracts to complete an electrical circuit
Explanation:
A thermostat has two bimetallic strips which is made up of two different metals in which one metal expand more than the other. This bimetallic strip acts as a bridge between an electrical and a heating system.
This bimetallic strip carries the electricity (as it connects the two system) and gets heated. Heating causes the strip to expand and thus with expansion the strip starts bending up to a point where it breaks open the circuit and thus the electricity is turned off.
As the circuit breaks down the bimetallic strip starts cooling and then it regains its original shape.
