Final answer:
If the PCO relay fails to operate when the LH tank is selected, it likely means the fuel pump will not work properly. This malfunction could inhibit the operation of the fuel tank crossfeed valve, essential for transferring fuel between tanks.
Step-by-step explanation:
If the PCO (Pressure Cut-Off) relay fails to operate when the LH (Left Hand) tank is selected, the system effected would be related to the fuel management in an aircraft. Assuming this question pertains to aviation and fuel systems, if the PCO relay fails, it is most likely that the fuel pump associated with the LH tank will not operate properly. This could have broad implications for fuel delivery and engine operation. However, the options given seem to be related to fuel crossfeed functions. In an aircraft fuel system, the PCO relay typically interacts with a fuel pump, whereas the crossfeed valve is a separate component that allows fuel to flow between tanks. If the question specifically inquires about the crossfeed operation and not just the pump, the correct answer would likely be that the fuel tank crossfeed valve will not work (option d). This would prevent fuel from being transferred from the LH tank to the RH tank or vice versa, which could be critical for balancing fuel loads or for continued engine operation if the other tank or pump fails.