a b c d

c

$\text{Consider a differential thickness dr at a radius r.}$ $\text{We get the volume for this differential thickness as } dV = 4\pi r^2 dr$ $\Rightarrow E.4\pi r_1^2 = \frac{Q}{\epsilon_0} = \frac{Qr_1^4}{\epsilon_0 R^4}$ $\Rightarrow E = \frac{Qr_1^2}{4\pi\epsilon_0 R^4}$