Concept: The question tests on basics of Number properties
Let x = AB/MN, where N is negative and x is positive.
For x to be positive, both Numerator and Denominator have to be both positive or both have to be negative.
Denominator is Negative: Then (a) M is positive and A positive, B negative or (b) M positive, A negative and B positive
Denominator is Positive: Then (a) M is negative and A positive, B positive or (b) M negative, A negative and B negative