Explanation:

Convection

In a car radiator, the coolant liquid flows through the tubes inside combustion engine and absorbs the engine's heat and in-turn gets heated itself.

The heated fluid then makes its way through a rubber hose to the radiator in the front of the car. As it flows through the thin tubes in the radiator, the hot liquid is cooled by the air stream entering the engine compartment from the grill in front of the car. Once the fluid is cooled, it returns to the engine to absorb more heat.

The air picks up heat from the coolant by convection process, i.e. heated air is blown away by the fan, and colder air replaces it.