Which basic machine element consists of a circular object that rotates around a central axis?

The correct choice is "wheel and axle" because this machine element specifically consists of a circular object (the wheel) that rotates around a central axis (the axle). This fundamental design allows the wheel to convert rotational motion into linear motion, or vice versa, which is a crucial component in various applications such as vehicles, machinery, and even simple tools.

Understanding the function of a wheel and axle is key in mechanics, as it enables the movement of loads with less friction compared to dragging something directly across a surface. This principle of reducing effort is leveraged throughout many mechanical systems, enhancing efficiency and utility.

Examining the other choices, a lever operates on a different principle of rotational force about a pivot point, while a pulley consists of a wheel with a groove for a rope but does not incorporate an axle in the same context. An inclined plane is a flat surface set at an angle, facilitating the movement of objects upwards or downwards but does not involve circular motion or axes at all. Thus, the wheel and axle stands out as the correct answer due to its unique characteristics that match the question's description.