Final answer:
When the opposite forces acting on an airplane are equal, the airplane's velocity is constant because there is no net force to cause an acceleration.
Step-by-step explanation:
If in an illustration the opposite forces acting on an airplane are equal in magnitude, it can be said about the airplane's motion that its velocity is constant. This is because when forces are balanced, there is no net force acting on the object, and therefore, according to Newton's first law of motion, the object will maintain its state of motion. If the plane had been accelerating prior to the forces becoming equal, it would continue to move at that constant velocity. Acceleration occurs only when there is a change in velocity, either in magnitude or direction. Since neither is mentioned as changing, the airplane is not accelerating.
The statement that the opposite forces acting on the airplane are equal in magnitude suggests that the airplane is in a state of equilibrium. In this case, the airplane's motion would be constant or without any acceleration. Therefore, the correct answer to the question would be option A: its velocity is constant.