Which of the following is NOT a type of key used in mechanical applications?

A snap key is not recognized as a standard type of key used in mechanical applications. In mechanical engineering, keys are essential components used to securely fit rotating parts, such as a shaft to a hub, ensuring they rotate together without slipping.

The square key is a common type that is square in cross-section, providing a strong and secure fit for various machinery. The Woodruff key, which has a semicircular shape, is used in applications where a more compact design is needed and allows for smoother rotation. The tapered key is another legitimate option, which provides a wedging action to secure the components more tightly when assembled.

In contrast, the term "snap key" does not correspond to any recognized type of mechanical key, which is why it stands out as the correct answer in this context.