Which factor can prevent an electric fuel pump from operating?

A bad fuel pump relay can prevent an electric fuel pump from operating because the relay is crucial for supplying power to the pump. When the ignition is turned on, the relay should close to allow electricity to flow to the fuel pump. If the relay is faulty or not functioning properly, it will not complete the electrical circuit needed to power the pump, thus preventing it from operating. This can lead to symptoms such as the engine not starting or stalling, as the pump is not providing the necessary fuel.

In contrast, the other factors listed would typically not hinder the operation of the electric fuel pump. Proper battery voltage ensures the pump has enough power to operate; a clean inline fuel filter is essential for maintaining fuel flow, and a functional ignitor switch is required for the ignition system. Each of these factors contributes to the optimal performance of the fuel system but does not block the fuel pump from operating like a faulty relay does.