Which type of machines have only one or two parts?

The correct choice is simple machines, which are characterized by having only one or two parts that work together to perform a specific task. Examples of simple machines include levers, pulleys, inclined planes, wedges, screws, and axles. These machines simplify work by providing a mechanical advantage, making it easier to lift, move, or change the direction of a force.

For instance, a lever consists of a rigid bar that pivots around a point called the fulcrum. This type of operation allows you to lift heavy objects using less effort, making certain tasks much more manageable.

In contrast, complex machines consist of multiple simple machines working together, which inherently increases their number of parts and mechanisms involved. Compound machines, a subset of complex machines, specifically involve two or more simple machines combined. Advanced machines typically refer to more modern or sophisticated technological systems which are considerably more complex than simple machines. Therefore, the defining feature of simple machines being composed of only one or two parts makes them unique and essential in the study of mechanics.