How do the engine's pistons primarily move?

The pistons in an engine primarily move up and down in a linear motion within their cylinders. This up-and-down movement is crucial for the engine's operation as it occurs in response to combustion within the cylinder, where the fuel-air mixture ignites and pushes the piston down. As the piston moves down, it creates a vacuum that draws in more fuel-air mixture, and then it moves back up to compress this mixture for the next combustion cycle.

The phrase "in and out" is often misunderstood, as this could imply a lateral movement, which is not the case for standard piston operation in most engines. However, during the up-and-down motion, the pistons do push the crankshaft "in and out" of the engine block in relation to their motion connecting to the crankshaft.

This movement is essential for converting the linear motion of pistons into rotary motion, which ultimately drives the vehicle's wheels. Thus, the correct choice highlights the fundamental up-and-down movement that defines a piston’s action in an internal combustion engine.