Which of the following is NOT a component of the engine's cooling system?

The fuel injector is not a component of the engine's cooling system. Instead, it is part of the engine's fuel delivery system, responsible for delivering the right amount of fuel to the combustion chamber for efficient engine operation.

In contrast, the other components listed play crucial roles in the cooling system. The water pump circulates coolant through the engine and radiator, ensuring that heat generated during combustion is effectively dissipated. Water jackets are passages built into the engine block that allow coolant to flow close to the engine cylinders, absorbing and transferring heat away from the engine to maintain optimal operating temperatures. The radiator serves to cool the heated coolant before it re-enters the engine, ensuring the system remains effective in regulating temperatures.

Thus, the correct answer is determined by recognizing that the fuel injector functions in a different capacity, entirely separate from the cooling processes that maintain engine temperature.