Final answer:
The drag force on a tennis ball that is hit straight up acts downward while the ball rises and continues to act downward as it falls back down, so the correct answer is always down.
Step-by-step explanation:
The direction of the drag force on a tennis ball hit straight up and then coming back down is always opposite to the direction of the ball's motion. When the ball is moving upward, the drag force acts downward; and when the ball is falling back down, the drag force still acts downward. Thus, the correct answer to the question is (c) always down. This is because drag is a type of frictional force acting opposite to the relative motion of any object moving with respect to a surrounding fluid.