answer: the gravitational pull on moon b is greater than on moon a because moon b is closer to the new planet than moon a.
the gravitational force exerted by the two objects on each other is inversely proportional to the square of the distance between the objects.
f = gravitational force or pull
g = gravitational constant on that planet
m = mass of the object-1
m = mass of object-2(mass of moon-a or moon-b)
r = distance between two objects
with decease in distance 'r' the force between the object increases or vice versa.so, from this we can say that the gravitational pull on moon-b is more than the gravitational pull on moon-a because the moon b is closer than the moon-a from the new planet.