Which tool is used for compressing rings for inserting into a cylinder?

The correct answer is the piston ring compressor, which is specifically designed for compressing piston rings so they can be easily inserted into the cylinder bore without causing damage. The tool works by encapsulating the rings and compressing them down to a smaller diameter, allowing for a smooth installation into the cylinder. This is crucial because piston rings need to fit snugly against the cylinder walls to maintain proper engine compression and lubrication.

The other choices do not serve this purpose; a pipe wrench is used for gripping and turning pipes and fittings, while a pipe union and pipe plug are plumbing components used to join or seal pipes. None of these tools are designed for handling or installing piston rings.