On the __________ stroke of an engine, the piston is going down, one valve is open and the other valve is closed and air and fuel are being drawn into the cylinder.

The correct choice is the intake stroke of an engine. During the intake stroke, the piston moves downward in the cylinder, creating a vacuum that allows air and fuel to be drawn into the combustion chamber. This process is facilitated by the opening of the intake valve while the exhaust valve remains closed, ensuring that only the fresh air-fuel mixture enters the cylinder. This phase is crucial for preparing the mixture necessary for combustion in subsequent strokes.

In contrast, during the compression stroke, the piston moves upward, compressing the air-fuel mixture. The power stroke occurs when the mixture is ignited, forcing the piston downward. Lastly, the exhaust stroke involves the expulsion of combustion gases from the cylinder, which occurs after the power stroke. Understanding these strokes in the four-stroke cycle of an engine is essential for diagnosing performance issues and for effective engine maintenance.