When one end of the rocker arm is pushed up, it pushes down on which component?

The correct choice is that the rocker arm, when pushed up on one end, exerts force downward on the valve stem. The rocker arm serves as a lever within the engine's valvetrain system, which is responsible for opening and closing the engine's valves.

When the rocker arm is actuated—typically by a camshaft rotating and making contact with one end of the rocker arm—it pivots at a fulcrum point. The movement of the rocker arm directly impacts the valve stem connected to the valve itself. As the rocker arm pushes down on the valve stem, it opens the corresponding valve, allowing air and fuel to enter the combustion chamber (in the case of the intake valve) or enabling exhaust gases to exit (in the case of the exhaust valve).

Understanding this mechanism is crucial in grasping how the timing and operation of an engine's valves are controlled, which is vital for engine performance and efficiency.