Which of the following is an example of a simple machine?

A lever is indeed a classic example of a simple machine. Simple machines are fundamental mechanical devices that change the direction or magnitude of a force, allowing for easier movement or lifting of objects. Levers work on the principle of applying a force at one point to lift or move a load at a different point. They consist of a rigid bar that pivots around a fixed point known as the fulcrum. By manipulating the positions of the load and the effort applied, levers can magnify force, making tasks like lifting heavy objects more efficient.

In contrast, the other choices represent more complex systems or devices that typically incorporate multiple simple machines or additional features, rather than being classified as simple machines themselves. An engine converts energy into mechanical motion and consists of complex parts; a robot is a versatile machine designed to perform tasks autonomously and involves various components beyond simple mechanics; and a generator converts mechanical energy into electrical energy, involving intricate designs and systems. These devices serve important functions but do not fit the definition of simple machines.