I think it is C.
We know we have 4 bills part of the annual expenses. Paying a bill in full represents a 1. So paying 100% should equal 4.
I have paid the following of bill A,B,C,D
A = 1/2
B = 1/2
C = 1/4
D = 3/4
That totals 8/4.
(8/4) / 4 = 2/4 = .5 * 100 = 50% of the annual bills paid.
This Is very clever problem.
The problem is testing your understanding of averages. As long as you choose the same numbers as plug-ins for C,D the outcome will be 50%, since 1/4 and 3/4 are completing. (i.e. C = 100, D = 100)
Only when you start pluging in diffrent numbers for C,D the outcome will be different then 50%. (i.e. C = 150, D = 100)
The writer of this problem assumes that a test taker with a good grip on averages will choose (E) either by useing algebra or logic, but a weaker test taker will plug in numbers (and in a hurry) and will choose the same numbers for C,D and eventually choose (C).