Step 1: Formula for moment of inertia of a solid sphere.
The moment of inertia of a solid sphere about its diameter is given by:
\[
I = \frac{2}{5} M R^2
\]
where \( M \) is the mass of the sphere and \( R \) is its radius. Step 2: Substitute mass.
The mass of the sphere is related to its density \( \rho \) by \( M = \rho \frac{4}{3} \pi R^3 \). So, substituting this in the formula for \( I \):
\[
I = \frac{2}{5} \left( \rho \frac{4}{3} \pi R^3 \right) R^2 = \frac{176}{105} R^2 \rho
\]
Thus, the correct answer is (3).