When does the process of pushing back an aircraft typically occur?

The process of pushing back an aircraft typically occurs before taxiing to takeoff. During this stage, ground crew members use specialized equipment to move the aircraft away from the gate and prepare it for its journey. This step is essential for ensuring that once the aircraft is cleared for taxi, it can successfully proceed to the runway without any obstructions.

While the other options involve various components of the boarding and pre-flight processes, they do not accurately describe when pushback occurs. For instance, boarding passengers is usually completed before pushback, as is refueling, which is typically finalized before any movement of the aircraft to ensure safety and efficiency. Finally, all passengers being seated does happen before the aircraft is pushed back, but this action is part of the boarding process rather than the pushback itself. Therefore, the answer that best captures the timing of the pushback procedure in relation to the other flight preparations is the time just prior to taxiing for takeoff.