You said it yourself... the phrase "900 miles at 30 miles per hour" means that the fraction 900/30=30 is showing how many hours it takes to do 900 miles at a speed of 30 miles an hour. So, it took 30 hours. As with your question, I am not entirely sure of what you are asking, but from the answer choices, the most logical would be the last one, 900 miles/ ? hours. If you implement the number of hours that it takes to drive 900 miles at 30 miles per hour, the fraction would be 900 miles/ ? hours, which would then show the rate that you are driving at, 30 miles per hour.
So your answer is 900/? hours or D. (on edge)