A pipe fitting which has both internal and external threads is the

The correct choice is bushing, which is a type of pipe fitting that features both internal and external threads. This design allows a bushing to adapt between different pipe sizes or types, connecting a male threaded pipe to a female threaded fitting. The internal threads enable it to attach to one pipe or fitting, while the external threads allow another pipe or fitting to screw onto it.

Understanding the function of a bushing is important in various plumbing applications where a reduction in pipe size or a transition between different connector types is required. This versatility makes bushings essential in many piping systems, ensuring a secure and leak-proof connection.

Other fittings mentioned serve different purposes: unions allow for easy disconnection of pipes, reducers decrease the diameter of a pipeline, and couplings join two pipes of the same diameter. Each of these fittings plays a specific role, but none provide the dual threading feature that a bushing does, highlighting the unique functionality of this specific fitting.