Two balls, each of radius R, equal mass and density are placed in contact, then the force of gravitation between them is proportional to:

let mass of two balls are $m_1=m_2=m\left(given\right)$  and the density be $\rho$ .
Distance between their centres =AB=2R
Thus, the magnitude of the gravitational force F that two balls separated by a distance 2R exert on each  other, is :
2R exert on each other, is:
$\begin{array}{l}\mathrm{F}=\mathrm{G}\frac{\left(\mathrm{m}\right)\left(\mathrm{m}\right)}{{\left(2\mathrm{R}\right)}^2}\\ =\mathrm{G}\frac{{\mathrm{m}}^2}{4{\mathrm{R}}^2}=\mathrm{G}\frac{{\left(\frac{4}{3}{\mathrm{\pi R}}^3\mathrm{\rho }\right)}^2}{4{\mathrm{R}}^2}\\ \therefore \mathrm{F}\propto {\mathrm{R}}^4\end{array}$