A moving part that fits closely in a cylinder to either pressurize a liquid or compress a gas is a:

A) piston
B) fulcrum
C) pushrod
D) jack handle

Piston

A cylindrical component that moves up and down in an engine's cylinder, converting combustion energy into mechanical motion.

Compress

To apply pressure to something or reduce its volume by squeezing it tightly, often referring to gases, fluids, or solids in various contexts.

Cylinder

A fundamental component of engines where fuel combustion occurs, generating power; it’s typically cylindrical in shape.