Answer
Gravitational forces:
$B\lt A \lt C \lt D$
Work Step by Step
According to Newton's universal law of gravitation
$F=G\frac{m_1m_2}{r^2}$
For system A $F_A=G\frac{mm}{r^2}=G\frac{m^2}{r^2}$
For system B $F_B=G\frac{m(2m)}{(2r)^2}=G\frac{m^2}{2r^2}$
For system C $F_C=G\frac{(2m)(3m)}{(2r)^2}=G\frac{3m^2}{2r^2}$
for system D $F_D=G\frac{(4m)(5m)}{(3r)^2}=G\frac{20m^2}{9r^2}$
Now, by comparing the above quantities, we can conclude that the forces rank as:
$B\lt A \lt C \lt D$
