What happens if there are only 3 cabin crew members on board before departure?

When there are only three cabin crew members on board before departure, boarding cannot commence until certain operational criteria are met, including refueling. This is primarily due to safety protocols established by aviation regulations. Ensuring that all cabin crew members are present before boarding starts is critical to maintaining safety and efficiency during the boarding process. The presence of adequate crew is crucial for managing passenger needs and emergencies that might arise, and thus operations cannot proceed in a manner that compromises these standards.

While the other options touch upon aspects of crew responsibilities or presence, they do not align with the regulatory requirements that dictate operational safety and readiness prior to departure, emphasizing the significance of having a full complement of crew before passengers are allowed to board.