B. 3 times the sum of a number and 4 is 3(n + 4).
(I took this course)
To find the answer all you needed to do is to look at the words. Therefore "three times" indicates that you will be multiplying by 3. And "sum of a number and 4" indicates that you will be adding n and 4. You are multiplying 3 by n plus 4. So B is the reasonable answer.