Which type of connector is commonly used for joining copper pipes?

Compression fittings are widely used for joining copper pipes because they offer a reliable and leak-proof connection without the need for soldering or welding. This type of fitting consists of a nut, a ring (or ferrule), and the fitting body itself. When the nut is tightened, it compresses the ferrule onto the copper pipe, creating a tight seal.

This method is particularly advantageous in situations where it may be difficult to solder, such as in tight spaces or in areas where using an open flame is not safe. Additionally, compression fittings can be disassembled easily, which allows for maintenance and alterations without damaging the pipes.

The other options have their specific applications, but compression fittings are the most commonly used for copper pipes in a variety of plumbing and heating systems, making them the correct choice for this question.