During which phase of operation should the integrity of the fuel system be most frequently checked?

The integrity of the fuel system should be most frequently checked during pre-flight preparations because this phase is critical for ensuring that all systems, including the fuel system, are functioning correctly before the aircraft departs. During pre-flight, thorough checks are conducted to confirm that the fuel levels are adequate, there are no leaks, and that all fuel system components are operating properly. This proactive approach allows for the identification of potential issues before encountering any operational phases where those issues could lead to serious consequences.

In contrast, while taxiing, during take-off, and in-flight checks may not provide the same thorough overview of the fuel system's integrity as those conducted during pre-flight preparations. These phases are more focused on the immediate handling and operation of the aircraft rather than detailed system checks. Therefore, pre-flight is the optimal time for ensuring the fuel system’s integrity, allowing for corrections before any flight operations begin.