Explanation

(d) Efficiency of a rectifier is given by
$\eta = \frac{DC}{AC}\frac{power\ output}{power\ input}$
For full wave rectifier
DC power output = $I_{DC}^{2} \times R_{L} = \left( \frac{{2I}_{0}}{\pi} \right)^{2} \times R_{L}$
AC input power = $I_{rms}^{2}\left( r_{f} + R_{L} \right) = \left( \frac{I_{0}}{\sqrt{2}} \right)^{2}\left( r_{f} + R_{L} \right)$
$\therefore$ Rectifier efficiency
$\eta = \frac{\left( \frac{{2I}_{0}}{\pi} \right)^{2}R_{L}}{\left( \frac{I_{0}}{\sqrt{2}} \right)^{2}\left( r_{f} + R_{L} \right)} = \frac{0.812{\ R}_{L}}{r_{f} + R_{L}}$
$\eta\ $will be maximum, if $r_{f}$ is negligible as compared to $R_{L}$.
$\therefore$ Maximum rectified efficiency = 81.2%.