Explanation

(c)
$\ \ \ \ {Cr}_{2}O_{7}^{2 -} + 14H^{+} + n{Fe}^{2 +} \longrightarrow 2{Cr}^{3 +} + n{Fe}^{3 +} + 7H_{2}O$
$\ \ \ {Cr}_{2}O_{7}^{2 -} + 14H^{+} + 6e^{-} \longrightarrow 2{Cr}^{3 +} + 7H_{2}O$ _{(reduction)...(i)}
${Fe}^{2 +} \longrightarrow {Fe}^{3 +} + e^{-}$ ^{(oxidation)…(ii)}
Eq.(ii)is multiplied by 6
$6{Fe}^{2 +} \longrightarrow 6{Fe}^{3 +} + 6e^{-}$
Thus, balanced equation is
${Cr}_{2}O_{7}^{2 -} + 14H^{+} + 6{Fe}^{2 +} \longrightarrow 2{Cr}^{3 +} + 6{Fe}^{3 +} + 7H_{2}O$
Hence, the value of ‘$n$’ is 6.