(c)
For the change , ${2Fe}^{3 +} + \ {3I}^{-}\ \rightleftharpoons \ {2Fe}^{2 +}\ + \ I_{3}^{-}$
$E_{cell}^{{^\circ}}$ = 0.77 – 0.54 = 0.23 V
$E^{{^\circ}} = \ \frac{0.059}{2}\log K_{c}\$
$0.23\ = \ \frac{0.0591}{2}\log K_{c}\ \$
$K_{c}\ = \ 6.26\ \times \ 10^{7}$