During taxi, takeoff, and landing, what must all persons in an airplane do?

The correct answer emphasizes the critical safety requirement of fastening seatbelts and shoulder harnesses during taxi, takeoff, and landing. This practice is essential because it helps to ensure that all occupants remain securely in their seats, minimizing the risk of injury during these critical phases of flight.

In the event of turbulence, sudden stops, or any kind of emergency, properly secured passengers are less likely to be injured or to interfere with the pilot's operations. Seatbelts and shoulder harnesses are designed to keep individuals safely in their designated seats, providing much-needed protection.

The other options propose actions that do not align with standard safety protocols. For example, keeping hands inside the cabin does not address the primary safety concern of secure seating. Remaining seated but unbuckled does not provide the necessary protection in the event of an emergency. Wearing a life jacket is specific to over-water flights and not relevant during taxi, takeoff, or landing in most scenarios.