Final answer:
The item designed to keep engine valves closed is the spring in the valve-train system of an engine. thus, the correct option is D).
Step-by-step explanation:
The item that is designed to keep engine valves closed is the spring. The spring is a crucial component of the valve-train system in an engine. It exerts force to keep the valves closed, allowing combustion gases to remain contained within each cylinder.
When the camshaft rotates, it pushes against the lifters, which then push against the pushrods. The pushrods, in turn, push against the rocker arms. Finally, the rocker arms cause the valves to open and close. However, in the absence of a force that keeps the valves closed, they would not completely seal, and the engine would not work efficiently.
The spring ensures that the valves close properly after the rocker arm lifts them during the opening phase. This mechanism prevents any backflow of gases from the cylinder into the intake or exhaust system. Springs in an engine valve system are usually made of high-strength steel, which can withstand the high temperatures and pressures present in the combustion chamber.