Your answer would be D: 74
This is because, if you go through it in your head, you get that the first floor has 20 offices, the second floor has (20 + 6) offices and the third floor has (20 + 6) + 2 offices. When you actually do those sums, you get 20, 26, and 28, and so when you add up all of those values, your final answer is 74.
I hope this helps!