What is one of the flight crew responsibilities when fueling with passengers on board?

One of the flight crew's key responsibilities during fueling operations with passengers on board is to ensure that the flight deck is manned by at least one pilot. This requirement is crucial for maintaining safety and operational control of the aircraft. Having a pilot present in the flight deck allows for immediate response to any situations that may arise during fueling, which could include emergency situations or security threats. The presence of a qualified crew member ensures that there is always someone who can take action if necessary.

In contrast, other options may not align with standard operating procedures for fueling with passengers. For instance, while keeping doors open might facilitate passenger evacuation in an emergency, it's not a requirement that directly supports fueling safety protocols. Similarly, while ensuring passengers are at the gate is a safety measure, it does not specifically address the responsibilities of the flight crew. Fueling before boarding is a general best practice aimed at minimizing risks, but it does not apply when the situation requires passengers to remain on board. Therefore, manning the flight deck reinforces the safety and operational integrity necessary during fueling with passengers on board.