Which of the following is a Pre-Departure duty?

The correct choice highlights a critical safety responsibility that is essential before an aircraft's departure. The installation of the steering bypass pin is a key pre-departure task performed by ground handling and ramp personnel. This ensures that the aircraft's steering system, specifically for towing and ground movement, is safely configured. The pin prevents accidental steering input while the aircraft is being towed on the ramp. This pre-departure task directly contributes to the safety of ground operations by ensuring that the aircraft can only be controlled in the manner intended during ground movements.

In contrast, while ensuring the cabin is secure, checking fuel levels, and completing flight plan documentation are all important tasks, they typically fall under different operational categories. Ensuring the cabin is secure is mostly about safety and cabin readiness; checking fuel levels is more related to ongoing operational readiness during fueling and before flight; and completing flight plan documentation generally pertains to documentation required for flight operations rather than specific pre-departure duties on the ground. Each of these is indeed vital for overall flight safety and operation, but installing the steering bypass pin is uniquely focused on the ground handling aspect of aircraft operation.