Which of the following is a redox reaction?

Assign $A$,$B$,$C$,$D$ from the given type of reaction.
$Pb_3O_4 + 8HCl \xrightarrow{\text{Warm}} 3PbCl_2 + Cl_2 + 4H_2O$

Balance the following equations in basic medium by ion-electron method and oxidation number methods and identify the oxidising agent and the reducing agent.
$(a)$ $P_{4(s)} + OH_{(aq)}^{-} \to PH_{3(g)} + HPO_{2(aq)}^{-}$
$(b)$ $N_{2}H_{4(l)} + ClO_{3(aq)}^{-} \to NO_{(g)} + Cl_{(g)}^{-}$
$(c)$ $Cl_{2}O_{7(g)} + H_{2}O_{2(aq)} \to ClO_{2(aq)}^{-} + O_{2(g)} + H^{+}$

In the equation,$BiO_3^{-} + 6H^{+} + xe^{-} \rightarrow Bi^{3+} + 3H_2O$. What is the value of $x$?

In the reduction of dichromate by $Fe(II)$,the number of electrons involved per chromium atom is:

The total change in the oxidation state of manganese involved in the reaction of $KMnO_4$ and potassium iodide in the acidic medium is $..........$.