Consider two balls of masses 100 kg and 200 kg and radii 0.1 m and 0.2 m respectively....

Consider two balls of masses 100 kg and 200 kg and radii 0.1 m and 0.2 m respectively. The gravitational force between them is 5 x 10^-7 N. What is the shortest distance between the surfaces of the two balls?

a) 0.20 m
b) 1.63 m
c)2.69 m
d)1.33 m

I used the formula Gravitational force = GM1M2/D^2 and plugged in the values
5 x 10^-7 = (6.67 x 10^-11)(100)(200)/d^2
solved for d= 1.63 which is b.
Is this the answer? I think I'm missing a step because I didn't do anything with the respective radii's.

The radii must be incorporated into the d^2 term, because d is the distance between the centres of mass of the two bodies, not their surfaces. In other words, in this case, you should break down d^2 into multiple components: the distance between the bodies' surfaces, plus the radius of each body.
5 x 10^-7 = (6.67 x 10^-11)(100)(200) / (x + 0.1m + 0.2m)^2
Solve this for x; that is the distance between the surfaces of the objects.