Heat can move by conduction, convection or radiation, and the dominant mechanism in a cooling system depends on how the heat is actually carried away from the engine. Here a moving liquid does the main work.

In a water-cooled diesel engine a pump circulates coolant through the water jackets, where it soaks up engine heat, and then on through the radiator tubes.

Heat carried from one place to another by a moving fluid is transferred by convection, and it is this circulating coolant that removes the bulk of the excess engine heat.

So the method most responsible for taking heat out through the radiator system is the convection of coolant through the radiator tubes.

Conduction only passes heat within the solid metal of the block and cannot move it out of the engine, air conduction is negligible, and radiation from the engine surfaces removes only a small fraction.

The flowing coolant is therefore the principal heat-carrying mechanism.

Hence, the answer is Convection of coolant through radiator tubes.