Which of the following is a required action during switching operations?

During switching operations, it is crucial that the air brake system is fully charged to ensure the safety and control of the train during these maneuvers. A fully charged air brake system helps maintain the necessary brake pressure to effectively stop or control movements of the train cars involved in switching. This is particularly important as switching often involves the coupling and uncoupling of cars, as well as moving them at low speeds, which requires precise braking capability to avoid collisions or accidents.

Other actions related to switching operations may be important, but they do not carry the same level of critical necessity as ensuring the air brake system is charged. For instance, while having all crew members present is generally good practice for communication and safety, it is not a strict requirement in every context of switching operations. Similarly, leaving switches unlocked or having signals set to proceed can lead to safety hazards, which is why these actions are typically not standard practice during switching operations.