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Definition of Terminal Speed:
Terminal speed (sometimes called terminal velocity) is the final speed to which an accelerating object (usually a falling object) can accelerate.
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Many things will change as an object falls. Success on this question requires that you think about changes which occur over the course of time. That is, you must think about how quantities like speed, air resistance, net force and acceleration change as time changes. Changes in quantities as time changes - that's a lot of change and the very thing that makes this question difficult.
As an object falls, its speed increases due to the downward net force and the downward acceleration. The air resistance force is an upward force which is dependent upon the object's speed. As speeds increase, the upward force increases as well. As this upward air resistance force grows in size, it begins to approach the same size as the downward gravity force. Thus, the growth in the air resistance force leads to a decrease in the net force and a decrease in the acceleration. Eventually, the upward air resistance force will equal the downward force of gravity. At this point, there is a balance of forces and no more acceleration. The speed will no longer increase. The object has reached terminal speed or terminal velocity.
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