Which of the following describes a lever?

A lever is accurately described as a rigid bar that pivots around a fixed point known as the fulcrum. This definition encompasses the essential characteristics of levers, showing how they work to gain mechanical advantage when force is applied. By positioning the fulcrum between the effort and the load, levers can amplify energy, making it easier to lift or move heavy objects. The arrangement of the lever system allows for variations in force and distance, which is fundamentally how levers function in practical applications.

The other options provided describe different types of simple or compound machines, but they do not accurately define a lever. For example, pulleys and ropes pertain to a different type of mechanism focused on changing the direction of force. Rotational to linear motion describes a diverse set of machines that do not specifically relate to levers. Lastly, the description involving wheels and axles refers to a distinct class of machines, demonstrating different functional attributes compared to levers. Hence, the focus on the rigid bar and its pivotal action around a fulcrum clearly delineates the lever's unique role in the family of simple machines.