Two identical solid spheres are placed in contact with each other. The force of gravitation between two spheres will be proportional to (R = radius of each sphere)

Gravitational force is the attractive force between two objects that is given by:
${\text{F}}_{ }=\text{G}.\frac{{\text{m}}_{\text{1}}{\text{m}}_{\text{2}}}{R_{ }^2}$
Where m₁ and m₂ are the masses of the two bodies, R is the distance between them and G is the universal gravitation constant.

In this problem, 
m₁=m₂  , As the spheres, are identical.

$\text{F}=\text{G}.\frac{{\text{m}}^2}{R^2}$ 

$\mathrm{Mass} \mathrm{of} \mathrm{spher} =\mathrm{Density}\left(\mathrm{\rho }\right) \mathrm{X} \mathrm{volume}$

$\text{F}=\frac{\text{G}{\left(\frac{4}{3}\pi {\text{R}}^3\rho \right)}^2}{{\left(\text{2R}\right)}^2}$

$\text{F}=\frac{\text{G}{\left(\frac{4}{3}\pi \rho \right)}^2.{\text{R}}^6}{{\text{4R}}^2}$

$\Rightarrow \text{F}\propto {\text{R}}^{\text{4}}$