Fuel is combusted in the stroke of a four-stroke engine.

a. intake
b. compression
c. power

( give explanation as well)

Fuel is combusted in the power stroke of a four-stroke engine since this is where ignition of the fuel occurs and this stroke makes the substance gases to thrust in contact with the pistons.

To add, the power stroke is the stage of the cycle of an internal combustion engine in which the piston is driven outward by the expansion of gases.