Which schematic symbol depicts energy conversion devices, such as an air motor, in a pneumatic schematic?

The correct choice for representing energy conversion devices, such as an air motor in a pneumatic schematic, is a circle. In pneumatic schematics, the circle is commonly used to indicate components that convert one form of energy into another, typically mechanical energy from air pressure in the case of an air motor. This symbolism is standardized in various pneumatic and hydraulic diagramming conventions, making it easily recognizable for those trained in system design and maintenance.

The circle symbol effectively communicates that the component is integral to the process of energy conversion, allowing technicians and engineers to quickly identify critical parts within the schematic. The other shapes mentioned—oval, square, and diamond—are designated for different types of components or functions in pneumatic diagrams, such as connectors, valves, or control devices, and therefore do not serve the same purpose as the circle in indicating energy conversion devices.