It would be C. An odd number of factors were negative.
We can start off by eliminating A. If all of the numbers were positive: 1 x 2 x 3 x 4 = 24, and 24 ÷ - 2 = -12, since you are still dividing a positive by a negative.
With B- Both problems have you multiplying negative numbers by each other until they cancel out. In B, if you multiply -1 x -2, the negatives cancel out and you get 2. and then, -3 x -4 = 12, negatives cancelling out again, and your last two numbers are going to be positive, so once you multiply them together and multiply by a negative your result will be negative.
D is similar to B. Since we have 4 numbers, two will have to be negative. -1 x 2 x -3 x 4 = 24 since the two negatives will cancel each other out.
the answer is C. An odd number of factors were negative then, since if we have 1 or 3 negative numbers, there will be one left over after the others cancel each other out.
Sorry if this explanation doesn't make much sense! if you would like it better explained let me know.
