This is a very badly worded question because it is ambiguous. As you correctly say, matrices can't be divided. The reason for that is that the "quotient" should mean the product of C and the inverse of A. But that could be either or . Those products are not the same, and so there are two possible solutions to the question.

If you take the equation (X+B)-\frac CA = 0 and multiply it on the left by A then you get , leading to the solution . But if you multiply the original equation on the right by A then it becomes , leading to the solution . Those two solutions are different, and they are both equally valid.