Explanation

(b)
Ferrous ion ($Fe^{2 +}$) changes to ferric ion ${Fe}^{3 +}$ on reacting with acidified $H_{2}O_{2}$ as.
$2K_{4}\lbrack Fe(CN)_{6}\rbrack + H_{2}{SO}_{4}\ H_{2}O_{2} \longrightarrow$
$\ \ \ \ \ \ \ \ \ \ \ \ \ \ \ \ 2K_{3}\lbrack Fe(CN)_{6}\rbrack + K_{2}{SO}_{4} + 2H_{2}O$
Electronic configuration of ${Fe}^{3 +} =$
$1s^{2},2s^{2},2p^{6},3s^{2},3p^{6},3d^{5}$
Number of $d$-electrons =5
Magnetic moment=$\sqrt{n(n + 2)}$
=$\sqrt{5(5 + 2)}$=5.92BM