Which of the following reactions involves oxidation-reduction?

The dark purple colour of $KMnO_4$ disappears in the titration with oxalic acid in acidic medium. The overall change in the oxidation number of manganese in the reaction is.

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

$A$ $20.0 \,mL$ solution containing $0.2 \,g$ impure $H_2O_2$ reacts completely with $0.316 \,g$ of $KMnO_4$ in acidic solution. The purity of $H_2O_2$ (in $\%$) is........... (mol. wt. of $H_2O_2 = 34$; mol. wt. of $KMnO_4 = 158$)

$A$ solution of $Na_2S_2O_3$ is standardized iodometrically against $0.167 \ g$ of $KBrO_3$ where $BrO_3^-$ changes to $Br^-$. This process requires $45 \ mL$ of the $Na_2S_2O_3$ solution. What is the strength of the $Na_2S_2O_3$ in $N$?
$[Mw \text{ of } KBrO_3 = 167]$

After balancing the equation,$C_2O_4^{2-} + H^+ + MnO_4^- \longrightarrow CO_2 + Mn^{2+} + H_2O$,the coefficient of $CO_2$ is: