What is the name of a fitting that closes the end of a pipe?

The fitting that closes the end of a pipe is referred to as a cap. Caps are specifically designed to seal the open end of a pipe, preventing fluid from escaping and keeping the interior of the pipe clean and protected from contaminants. This is particularly important in various plumbing and piping systems, where maintaining pressure and preventing leakage are critical.

The other options represent different types of fittings that serve distinct functions. For instance, a reducer is used to connect pipes of different diameters, facilitating a transition in pipe size. An elbow fitting changes the direction of the pipe, typically at a 90-degree or 45-degree angle, allowing for bends in the piping system. A coupling is a connector that joins two lengths of the same or different diameter pipes, enabling the extension or modification of piping runs. Each of these fittings plays an important role in a piping system, but they do not function to close the end of a pipe like a cap does.