In which reaction is there a change in the oxidation state?

Observe the following reaction:
$aP_4{_{\text{(s)}}} + bOH^{-}{_{\text{(aq)}}} + cH_2O_{\text{(l)}} \rightarrow dPH_3{_{\text{(g)}}} + eH_2PO_2^{-}{_{\text{(aq)}}}$
The values of $a, b, c,$ and $d$ are respectively:

What is the number of electrons gained by $Cl$ atom when $n$ mole of $ClO_4^-$ is transformed into $n$ mole of $ClO_2^-$ ion?

In alkaline medium,the reduction of permanganate anion involves a gain of $........$ electrons.

If an iron rod is dipped in $CuSO_4$ solution,what happens?

Permanganate $(VII)$ ion,$MnO_4^-$,in basic solution oxidises iodide ion,$I^-$,to produce molecular iodine $(I_2)$ and manganese $(IV)$ oxide $(MnO_2)$. Write a balanced ionic equation to represent this redox reaction.