What should passengers do once supplemental oxygen masks drop?

When supplemental oxygen masks drop, passengers should pull the mask firmly to start the oxygen flow. This action is essential because the masks require a firm tug to activate the oxygen delivery system, ensuring that the passenger receives the necessary supplemental oxygen in case of cabin depressurization. Prompt action is crucial in emergency situations to ensure personal safety, and the masks are designed to be easy to use once the flow of oxygen starts.

The other options suggest actions that are either unnecessary or incorrect in the context of using the oxygen masks. For instance, securing the masks before use would delay receiving oxygen, which is counterproductive in an emergency. Waiting for instructions from the crew could lead to longer exposure to unsafe conditions if immediate action is required. Additionally, securing the mask around the neck is unsafe and ineffective as it does not provide the required oxygen flow to the passenger. Therefore, the immediate focus should be on pulling the mask to initiate oxygen flow.