How much fuel is required to be on board an aircraft?

Having sufficient fuel on board an aircraft is critical for ensuring safety and compliance with aviation regulations. The correct choice emphasizes the need to have enough fuel to complete the flight to the intended landing base or an alternate airport if necessary. This is crucial in cases where unexpected circumstances, such as weather changes or air traffic delays, necessitate diverting from the original destination.

In addition to reaching the destination, the answer also mentions the requirement for reserve fuel. The reserve is an essential safety margin that allows the aircraft to handle emergencies or unpredictable situations without running the risk of fuel exhaustion. This aligns with regulations set forth by aviation authorities, which require not only the fuel for the planned flight but also additional fuel to accommodate unforeseen contingencies.

In contrast, the other options do not account for the complete safety protocol required for flight. Simply estimating the fuel needed for the flight without considering reserves or alternates can lead to dangerous situations. Filling the tanks to capacity does not guarantee a sufficient fuel supply for flight requirements, and a minimum of 50% does not ensure the aircraft can handle any unexpected scenarios or require diversions. Thus, option A establishes a standard that prioritizes safety through comprehensive fuel planning.