Final answer:
The special type of nut serving the function of a tapped hole is called a Rivet nut. Rivet nuts provide a threaded receptacle in materials too thin for standard tapped holes or when backside access is limited.
Step-by-step explanation:
A special type of nut serving the function of a tapped hole is called a Rivet nut. Rivet nuts are a type of fastener that can be installed into materials such as plastics, metals, or composites to provide a threaded receptacle for a machine screw or bolt. They are particularly useful when the material in question is too thin to allow the creation of a standard tapped hole, or when there might not be access to the back side of the material, as commonly encountered in sheet metal work or tube interiors.
Other nuts listed, such as the Acorn nut, T-nut, and Barrel nut serve different functions. An Acorn nut is a cap nut often used for aesthetic purposes to cover exposed threads; a T-nut is inserted into wood and has a flange at one end to provide a large mounting surface; and a Barrel nut is a cylindrical shaped nut used to fasten two pieces of material together with a bolt, often in furniture construction.