What is the pipe fitting with an outside thread and projecting square head used for closing the edge of another fitting?

The correct choice is the plug because it specifically refers to a type of pipe fitting that is designed to seal the end of a pipe or close the opening of another fitting. The defining characteristic of a plug is its outside threading combined with a projecting square head, which allows for easy installation and removal using a wrench. This feature facilitates securing the fitting tightly and ensures there are no leaks at the closure point.

In contrast, other options serve different purposes: a cap is typically used to close the end of a pipe but does not have a square head; a knot does not pertain to plumbing fittings; and a bushing is utilized to connect pipes of different sizes, often with internal threads, rather than to close an opening. Thus, the anatomical design and intended function confirm that the plug is indeed the correct answer for closing another fitting's edge.