Which of the following steps is involved in locking the main door of the aircraft?

The step involved in locking the main door of the aircraft is accurately described by the correct answer. Depressing the gust lock located on the door hinge is an essential action that ensures the door is securely locked in place. The gust lock prevents the door from moving unexpectedly due to aerodynamic forces, ensuring safety during ground operations and preventing any potential damage or accidents.

While other actions, such as grasping the door assist handle and pulling the door outwards, may be part of the general operation of opening or closing the door, they do not specifically pertain to the locking mechanism. Similarly, rotating the operating handle and pulling the door outwards are more related to the door's operation rather than securing it in a locked position. Understanding the significance of the gust lock is crucial for ensuring that all safety measures are followed during aircraft operations, which directly contributes to passenger safety and aircraft integrity.