Explanation

(d) We know that plate resistance is given by
${\ r}_{p} = \frac{\delta V}{\delta I}$
where $\delta V\ $is change in voltage and $\delta I$ is change in current.
Also, at saturation change in current is zero.
$\therefore r_{p} = \frac{\delta V}{0} = \infty$
Hence, plate resistance will be infinite.