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

The correct answer is "D. Gear" because gears are considered complex machines rather than simple machines. The six simple machines are lever, pulley, wedge, inclined plane, screw, and wheel and axle. These simple machines are fundamental devices that use mechanical advantage to perform work and can be combined to create more complicated machinery, such as gears.

Gears function by transferring motion and force between different components, typically involving rotations. While they are crucial in the operation of many mechanical systems and are derived from the principles of simple machines, they themselves cannot be classified as one of the six simple machines. Understanding the distinction between simple machines and more complex systems like gears is vital in the study of mechanics and engineering.