Explanation

(c)
$2{Fe}^{3 +}\ + \ Zn\ \rightarrow \ {Zn}^{2 +}\ + \ 2\ {Fe}^{2 +}\ \$
$2{Fe}^{3 +}\ + \ {2e}^{-}\ \rightarrow \ 2\ {Fe}^{2 +}\ \$
According to Nernst’s equation,
$E_{cell}^{{^\circ}} = \ E_{cell}^{{^\circ}} - \ \frac{RT}{nF}In\ \frac{\lbrack{Fe}^{2 +}\rbrack^{2}}{\lbrack{Fe}^{3 +}\rbrack^{2}}\$
$E_{cell}$ is depend on concentration of ${Fe}^{2 +}$.
Therefore on increasing the concentration of $\lbrack{Fe}^{2 +}\rbrack E_{cell}$ decreases.