for box A :
gravitational pull is given as
F = G (10) (10) /r²
F = 100 G/r² eq-1
for Box B :
gravitational pull is given as
F' = G (10) (5) /r'²
F' = 50 G/r'² eq-2
for Box C :
gravitational pull is given as
F'' = G (10) (15) /r''²
F'' = 150 G/r''² eq-3
from the diagram , we see that
r = r'
hence F > F'
gravitational pull of box B is greater than gravitational pull of box A.
r'' > r = r'
such that r'' is almost 2 times of r and r'
so F'' = 150 G/(2r'²)
F'' = 37.5 G/r'² = 37.5 G/r²
hence box A has greatest gravitational pull between the objects.
A) the object inbox A would have the strongest gravitational pull because they are close to each other and have more mass.