Which of the following describes positive displacement pumps?

Positive displacement pumps are characterized by their mechanism of moving fluids through the action of a piston, diaphragm, or similar device that creates a specific volume of liquid to be displaced with each cycle or operation of the pump. This mechanism allows them to deliver a consistent flow rate, regardless of the resistance encountered in the system, making them particularly effective for applications requiring precise flow control.

These pumps operate by trapping a fixed volume of fluid and then forcing it into the discharge line, ensuring a steady and predictable output. This distinguishes them from other types of pumps, such as centrifugal pumps, which rely on rotational energy and pressure differences to move fluid and are sensitive to changes in system resistance.

In contrast, options describing pumps that use gravity (which relies on the natural force acting on fluid) or centrifugal action (which relies on the rotation of an impeller) do not accurately define positive displacement pumps. Furthermore, the use of electrical current is not a characteristic specific to positive displacement pumps, as many types of pumps can be electrically powered but operate on different principles.