Current Electricity question for NEET / JEE Main, with solution

Answer: (A) $R = r$

$P = \dfrac{E^2R}{(R + r)^2}$ is maximum when $\dfrac{dP}{dR} = 0$, which gives $R = r$. Then $P_{max} = \dfrac{E^2}{4r}$.

Solution by Sreeraj P, M.Sc Physics