Compression fittings used with copper or plastic tubing seal by means of a:

The correct answer involves understanding the sealing mechanism of compression fittings used with copper or plastic tubing. Compression fittings create a tight seal by relying on the design of their components, and it is the compression ring that plays a critical role in this process.

When a compression fitting is assembled, the nut is tightened, which causes the compression ring (or ferrule) to squeeze down onto the tubing. This action creates an intimate seal between the fitting and the tubing, preventing leaks. The design ensures that the connection is not only secure but also allows for the flexibility to disassemble and reassemble without damaging the tubing itself.

The other options do not accurately describe the function of compression fittings. A beveled sleeve, while related to fitting designs, does not specifically provide a sealing mechanism in the same way. Compressed beveled gaskets and o-rings also do not typically participate in the sealing process for compression fittings; instead, they are more relevant to different types of fittings or sealing applications in plumbing. Thus, recognizing the role of the compression ring clarifies why it is essential to the functioning of compression fittings.