This is a divisibility problem. First, notice that the number of guests must be divisible by 2,3, and 4: no fractional bowls are allowed due to the problem's nature. We can thus immediately eliminate A & C.

Turning to the remaining answers, let's check B & D; the number of bowls consumed by the guests must equal 91 as given in the problem.

For B: 72/4 + 72/3 + 72/2 = 18 + 24 + 36 = 78 < 91
For D: 84 * (1/4+1/3+1/2) = 84*(13/12) = 7*13 = 91 ( I did this one differently to test my mental math.)

The answer is D.