See the attraction in this question.

Example: 4 x 4 and 6 x 2, both of sum 8 but product of 16 and 12 respectively. Obviously, the one with same mass must produce higher value.

I would say that the only free variable is M and m, but we have M+m constant. By AM-GM, we have Mm maximised when M = m, so obviously the answer is the case B.

Mathematically speaking. Let the equal masses in case B be a+b and let 2a and 2b be the masses of two bodies in case A where a is not b(observe that the sum of the masses in each case is the same). Product of masses in case B is $(a+b)^{2}$ while it is 4ab in case A. Mathematically $(a+b)^{2}=4ab+n$ where n is a positive no.Because the equation can be simplified into $a^{2}+b^{2}+2ab=2ab+2ab+n$ $a^{2}+b^{2}-2ab=n$ $(a-b)^{2}=n$ As all squares of real no.s except 0 is positive, n is positive. That is $(a+b)^{2}>4ab$ Then according to sir Newton's equation for gravitational force more force is present in case B.