Final answer:
The correct description for the airplane's motion affected by the wind is that both are experiencing uniform linear motion in different directions.
Step-by-step explanation:
To describe the motion of the airplane and wind that affects it: The airplane is flying due east at a speed of 900 km/h. When hit by a crosswind blowing from the northeast at 50 km/h, the motion of the airplane deviates from its straight-line path. Considering the wind's influence, the correct description of the motions of both the airplane and the wind is represented by Option 2: Both the plane and the wind are experiencing uniform linear motion in different directions. The wind is moving in a straight line from the northeast, affecting the plane's trajectory, and the plane's path becomes a resultant vector of its original velocity with that of the wind's push.