d
Electric filed, $E=-\frac{d \phi}{d r}=-2 a r$
By Gauss's law, $E .4 \pi r^{2}=\frac{q_{i n}}{\in_{0}}$
$\Rightarrow q_{i n}=(-2 a r) 4 \pi r^{2} \in_{0}=-8 \pi \in_{0} a r^{3}$
Now $\frac{d q_{i n}}{d r}=-24 \pi \in_{0} a r^{2}$ and $V=\frac{4}{3} \pi r^{3}, \frac{d V}{d r}=4 \pi r^{2}$
Charge density, $\rho=\frac{d q_{i n}}{d V}=\frac{d q_{i n}}{d r} \times \frac{d r}{d V}=\left(-24 \pi \in_{0} a r^{2}\right) \times \frac{1}{4 \pi r^{2}}=-6 \in_{0} a$