Solution

(A) $$\text{In acidic medium, one mole of permanganate ion gains 5 moles of electrons whereas 1 mole of iron (II) ion}$$ $$\text{loses one mole of electrons.}$$ Hence, $$1$$ mole of $$MnO_4$$​ ion can oxidize $$5$$ moles of $$Fe^{2^+}$$ ion in acidic medium. $$\text{(B) In acidic medium, 1 mole of dichromate ion gains 6 moles of electrons whereas 1 mole of iron (II) ion loses}$$ $$\text{one mole of electrons.}$$ Hence, $$1$$ mole $$Cr_2​O_7^{2-}$$​ ion can oxidize $$6$$ moles of $$Fe^{2+}$$ ion in acidic medium. $$\text{(C) In acidic medium, one mole of permanganate ion gains 5 moles of electrons whereas 1 mole of }Cu_2​S$$ $$\text{loses two moles of electrons.}$$ Hence, $$1$$ mole of $$MnO_4$$​ ion can oxidized $$\dfrac52$$​ moles of $$Cu_2​S$$ ion in acidic medium. or 1 mole of $$Cu_2S$$ can oxidised by $$0.4$$ moles of $$MnO^−_4$$ ion in acidic medium. $$\text{(D) In acidic medium, 1 mole of dichromate ion gains 6 moles of electrons whereas 1 mole of }Cu_2​S$$ $$\text{loses two moles of electrons.}$$ Hence, $$1$$ mole $$Cr_2​O_7^{2-}​$$ ion can oxidized $$3$$ moles of $$Cu_2​S$$ ion in acidic medium. or 1 mole of $$Cu_2S$$ can oxidised by$$0.33$$ moles of $$MnO_4^-$$ ion in acidic medium. Option A,B are correct.