Well, we can eliminate the first 2 options as an absolute value of either a positive or negative number will yield a positive number. The absolute value of 0 is 0. This there is only 1 solution for this, and that is 0. The solution would be C.
It can't be D, because in both cases the absolute value of -1 and 1 will be 1 and thus there are 2 solutions, -1 and 1.