Explanation

(d) The critical frequency of sky wave undergoing reflection from a layer of ionosphere is
$f_{c} = 8\ \sqrt{N_{\max}}$
Where $N$ is electron density per $m^{3}.$
$N_{max\ } = \frac{f_{c}^{2}}{81} = \frac{\left( 10 \times 10^{6} \right)^{2}}{81} = 1.2 \times 10^{12}\ m^{- 3}$