In acid solution, potassium permanganate oxidises iron(II) ions to ir…

The correct answer is Option (2) → 100 mL

In acid solution each MnO₄⁻ ion takes up five electrons and is reduced to Mn²⁺, while each Fe²⁺ ion gives up only one electron. So one MnO₄⁻ oxidises five Fe²⁺ ions:

    \( \displaystyle \ce{MnO4^- + 8H+ + 5e^- -> Mn^2+ + 4H2O} \)

    \( \displaystyle \ce{5Fe^2+ -> 5Fe^3+ + 5e^-} \)

    \( \displaystyle \ce{5Fe^2+ + MnO4^- + 8H+ -> Mn^2+ + 4H2O + 5Fe^3+} \)

    \( \displaystyle n(\ce{Fe^2+}) = 0.0250\ \mathrm{L} \times 0.40\ \mathrm{mol\ L^{-1}} = 0.010\ \mathrm{mol} \)

    \( \displaystyle n(\ce{MnO4^-}) = \frac{0.010\ \mathrm{mol}}{5} = 0.0020\ \mathrm{mol} \)

    \( \displaystyle V = \frac{0.0020\ \mathrm{mol}}{0.020\ \mathrm{mol\ L^{-1}}} = 0.10\ \mathrm{L} = 100\ \mathrm{mL} \)

NCERT (2026-27 edition):
4.4.1 Oxides and Oxoanions of Metals – “1. In acid solutions:” … “(b) Fe²⁺ ion (green) is converted to Fe³⁺ (yellow): 5Fe²⁺ + MnO₄^– + 8H⁺ → Mn²⁺ + 4H₂O + 5Fe³⁺”