Which correctly describes a screw?

A screw is indeed described as an externally threaded fastener designed to fit into a threaded hole. This characteristic is fundamental to its function: the external threads are what enable the screw to be driven into a material, engaging with internal threads in a corresponding hole or another object. This threaded design allows for a strong mechanical connection, as the threads create friction that helps hold the screw securely in place.

The ability of screws to engage with threaded holes makes them versatile for a wide range of applications across different materials, including metal and plastic, not just limited to wood. This flexibility in usage is a key aspect that distinguishes screws from other types of fasteners.

In contrast, the other options present inaccuracies. For instance, a fastener with a smooth cylindrical body lacks the threads that are essential for the functioning of a screw. Also, an internally threaded fastener with no head would describe a different type of fastener altogether, such as a nut, rather than a screw. Lastly, claiming that screws are exclusively used for wood ignores their extensive applications in various fields, including construction and machinery, where they can fasten materials of all kinds.