Which type of fire pumps requires priming with an external priming device?

The correct answer is based on the operational characteristics of centrifugal pumps. Centrifugal pumps are designed to move fluids by converting rotational energy from a motor or engine into hydrodynamic energy. For these pumps to function properly, they need to be filled with liquid to create the necessary suction that lifts the fluid into the pump. In some cases, especially when the pump is located above the surface of the water, an external priming device is necessary to remove air from the pump casing and allow the fluid to enter the pump.

Positively displaced pumps, on the other hand, can move fluids regardless of the fluid level and do not typically require external priming devices because their mechanism relies on trapping a fixed amount of fluid and pushing it through the system. Vertical turbine pumps, while sometimes requiring priming, often operate at depths where gravity helps to fill the suction line. Diaphragm pumps can handle some level of air but are not usually categorized with the need for external priming like centrifugal pumps. Thus, the specific requirement of centrifugal pumps for external priming devices makes them the correct answer for this question.