Which type of screw does not have a head?

The stud is a type of fastener that does not have a head, making it the correct answer. Studs are typically used to attach two objects together, with threads on both ends allowing nuts to be secured on either side. This design provides a strong fastening method for applications where a head would interfere with assembly or where a flush surface is required.

In contrast, machine screws, wood screws, and cap screws all feature heads which serve various purposes, such as providing a gripping surface for a screwdriver or wrench, preventing the screw from being driven all the way through the material, and distributing the load more evenly across a surface. Each of these screw types is designed for specific applications that necessitate a head, distinguishing them from studs.