The number of electrons required to reduce chromium completely in $Cr_2O_7^{2-}$ to $Cr^{3+}$ in acidic medium is:

Balance the following redox reactions by the ion-electron method:
$(a)$ $MnO_{4(aq)}^- + I_{(aq)}^- \to MnO_{2(s)} + I_{2(s)}$ (in basic solution)
$(b)$ $MnO_{4(aq)}^- + SO_{2(g)} \to Mn_{(aq)}^{2+} + HSO_{4(aq)}^-$ (in acidic solution)
$(c)$ $H_2O_{2(aq)} + Fe_{(aq)}^{2+} \to Fe_{(aq)}^{3+} + H_2O_{(l)}$ (in acidic solution)
$(d)$ $Cr_2O_7^{2-} + SO_{2(g)} \to Cr_{(aq)}^{3+} + SO_{4(aq)}^{2-}$ (in acidic solution)

In neutral medium,potassium permanganate oxidises $I^{-}$ to $X$. Identify $X$.

What is the value of $n$-factor of $[Fe(CN)_6]^{-4}$ in the given reaction?
$[Fe(CN)_6]^{-4} \xrightarrow{MnO_4^-/H^{+}} Fe^{+3} + CO_2 + NO_3^-$

For the balanced redox reaction $a C_2O_4^{2-} + b MnO_4^- + c H^+ \rightarrow x Mn^{2+} + y H_2O + z CO_2$, the values of $a$ and $x$ respectively are:

Assign $A, B, C, D$ from given type of reaction.
$CuSO_4(aq.) + Zn(s) \longrightarrow ZnSO_4(aq.) + Cu(s)$