While flying the crosswind leg of a rectangular course, the airplane must be:

When flying the crosswind leg of a rectangular course, the airplane must be crabbed into the wind to maintain a stable flight path. This technique involves angling the aircraft's nose slightly into the wind so that the airplane can counteract the wind's lateral push. The objective is to ensure that the aircraft continues to fly straight along the designated course without drifting off to one side due to the wind.

Crabbing into the wind helps compensate for any crosswind component, enabling the pilot to stay aligned with the desired track over the ground. Maintaining this approach is crucial for precision flying during maneuvers, especially when preparing for takeoffs or landings.

When considering the context of the other options, flying level with the horizon does not account for the wind's effect, which would lead the aircraft off course. Banking away from the wind could create a situation where the aircraft is constantly being pushed away from the intended track. Cocking the airplane to the left, without understanding the wind direction, does not specifically address how to counteract the crosswind effectively. Therefore, crabbing into the wind is the most appropriate technique to ensure accurate navigation while in the crosswind leg.