The point $\;r_{m}\;$ inside the sphere of a sphere of radius R at which electric field intensity is maximum , is given by

Question

$(a)\;r_{m}=\large\frac{R}{3}\qquad(b)\;r_{m}=\large\frac{3R}{2}\qquad(c)\;r_{m}=\large\frac{2R}{3}\qquad(d)\;r_{m}=\large\frac{4R}{3}$