Final answer:
An airplane flying in a horizontal circle maintains a constant speed but not constant velocity since the direction of the velocity changes as the plane turns. Mach 1, terminal velocity, and variable speed are not the correct terms to describe the motion of the airplane in this context.
Step-by-step explanation:
The question pertains to the motion of an airplane flying in a horizontal circle. When an airplane is flying in a horizontal circle, it may maintain either a constant speed or a variable speed, depending on the specifics of its flight path and pilot inputs. Mach 1 refers to the speed of sound and is a measure of speed relative to the speed of sound, not necessarily indicative of the nature of the motion in terms of acceleration or velocity changes. Constant velocity implies both constant speed and constant direction, which is not possible in circular motion because even if the speed remains the same, the direction is always changing, hence the velocity changes owing to its vector nature. When speaking of a variable speed, it implies changes in the magnitude of the speed as the airplane flies the circle. Lastly, terminal velocity is a term generally used in the context of an object falling under gravity and reaching a constant speed due to air resistance; it is not applicable to an airplane flying in a horizontal circle. Therefore, the most appropriate answer is that an airplane flying in a horizontal circle would be at a constant speed while its velocity changes direction continuously.