During a planned emergency landing, what is the proper position to assume?

The brace position with palms facing down is the correct choice to assume during a planned emergency landing. This position is critical because it helps to stabilize the passenger's body against the forces of impact. By bracing with the head down and arms secured, it minimizes the risk of injury to the head and neck during a sudden stop or collision.

Additionally, having the palms facing down enables the individual to place their arms against their thighs or the seat in front, which can help provide extra stability. This position also keeps the body as compact as possible, reducing the risk of flailing limbs that could result in injury during an emergency landing.

In contrast, other positions such as seated upright with arms crossed or leaning back in the seat do not provide the same level of protection. The relaxed position lacks the necessary tension in the muscles to brace for impact, which could lead to more severe injuries upon landing. Hence, the brace position is explicitly designed for safety during such critical moments, making it the correct option.