The mean free path of molecules of a gas (radius ‘ $r$ ’) is inversely proportional to:

Here we have to calculate that mean free path of molecule is inversely prooportional to what power of r

we know that, 

Mean free path $l=\frac{1}{\sqrt{2}n{d}^2}$
So: $l\propto \frac{1}{d^2}\Rightarrow l\propto \frac{1}{r^2}$

Hence the mean free path of the molecule is inversely proportional to $r^2$.