Answer:
C < 12
Explanation:
The upper bound for C will be found where 'b' is a maximum and 'a' is a minimum.
The maximum that 'b' can be is half of the rounding value more than the reported value: 15 +5/2 = 17.5.
Similarly, the minimum 'a' can be is half the rounding value less than the reported value: 6 -1/2 = 5.5
Then the maximum value of C is ...
C = 17.5 -5.5 = 12
The upper bound for the value of C is 12. (C < 12)
_____
Additional comment
The minimum value of C will be (15 -5/2) -(6 +1/2) = 6. (6 < C). The value of C cannot actually be either of these values, since that would mean one of 'a' or 'b' would be rounded differently. It can only come close: 6 < C < 12.