The answer would be D. 200 - 20d.
This is because the question asks for pages left after reading for "d" (however many) hours. Since the book has 200 books, you'd subtract from that number since it's the "starting number" of pages. If he reads 20 pages per hour, you would multiply those together to get how many he has read, then subtract that number from 200, to get the amount of pages left.
I hope this helps!
