Which of the following is not one of the six simple machines?

The correct answer is "C" because a piston is not classified as one of the six simple machines. The six simple machines are the lever, wheel and axle, pulley, inclined plane, wedge, and screw. These machines are basic mechanical devices that change the direction or magnitude of a force, making work easier in various applications.

A piston, on the other hand, is a component typically found in engines and other machinery. It works in conjunction with other parts to convert pressure into mechanical work but does not fit the definition of a simple machine. The essence of simple machines is that they provide a mechanical advantage through straightforward mechanisms, while a piston involves more complex interactions of motion and forces within a confined space.

In summary, the reason "C" is the correct answer lies in the distinction between basic mechanical devices used for force manipulation (simple machines) and components of more complex systems, like the piston.