If a transfer failed on GEN 1, what is the probable cause?

The most appropriate answer is that the Generator Control Unit (GCU) prevented the transfer due to a fault. The GCU plays a crucial role in managing the operation of generators, including recognizing any anomalies or faults that could compromise safety or operational efficiency. If a fault is detected, the GCU will automatically prevent the transfer to protect the system from potential damage or failure.

Power loss to the generator could lead to a transfer failure, but it is not as direct of a cause as the GCU's intervention. The manual override might obstruct the transfer, yet it typically would not be seen as a probable cause unless specifically engaged. Insufficient fuel supply relates primarily to the operational status of the engines rather than the generator's transfer process, making it less relevant to this context. Hence, the GCU is the most likely reason for the transfer being blocked due to its protective functions.