Why are valves more critical in a one-cylinder engine than in a multi-cylinder engine?

In a one-cylinder engine, the valves are critical because they directly control the flow of air and fuel into the combustion chamber and the exhaust gases out of it. In a multi-cylinder engine, if one cylinder's valves fail, the loss of power is typically isolated to that one cylinder. The other cylinders can still contribute to the overall power output of the engine, allowing it to continue operating, albeit at reduced efficiency and power.

Conversely, in a one-cylinder engine, if the valves fail, the engine will not function at all. This is because that single combustion process depends entirely on the proper operation of its valves. The failure of the valves in this engine type means there is no intake of air and fuel, or no exhaust of combustion gases, which results in a complete loss of power and an inability to start the engine.

The other options do not convey the critical nature of valves in a one-cylinder engine as effectively as the concept that a failure in a multi-cylinder engine allows it to continue functioning at diminished capacity, whereas in a one-cylinder engine, such a failure results in total incapacitation of the engine.