What term describes a mechanical device that helps perform work?

The term that describes a mechanical device that helps perform work is "machines." Machines are defined as any device that uses energy to perform a specific task or to manipulate materials. They can range from simple tools, like levers and pulleys, to complex systems such as engines and robots.

Machines function by enhancing efficiency in performing tasks, allowing humans to accomplish work that would be difficult or impossible to do manually alone. For example, a car engine converts fuel into movement, demonstrating the principle of machines facilitating work through energy conversion and mechanical advantage.

While tools, engines, and instruments may also be involved in performing work, they are subsets of the broader category of machines. Tools typically refer to handheld devices used for specific tasks, engines generally denote specific types of machines that convert energy into motion, and instruments are usually used for measurement or control rather than directly performing physical work. Therefore, "machines" is the most comprehensive and correct choice in this context.