Which statement is true about a fuel pressure relief valve on an aircraft?

The statement that a fuel pressure relief valve is only required on positive displacement fuel pumps is accurate because of the operational characteristics of these pumps. Positive displacement pumps can generate significantly high pressures and, if not adequately managed, this can lead to potential damage within the fuel system. The relief valve plays a critical role by allowing excess fuel to bypass, thereby preventing overload and maintaining a safe operating pressure.

In contrast, centrifugal fuel pumps typically do not require a relief valve due to their inherent design and operational principles, which allow them to manage pressure more effectively by varying flow with the engine demand. This makes the relief valve unnecessary in those systems, as they do not experience the same risks of excessive pressure that positive displacement pumps do. Hence, option B correctly identifies the specific need for a pressure relief valve in the context of positive displacement pumps, emphasizing the importance of safety and performance in aircraft fuel systems.