In the preparation of chlorine from $HCl,$ $MnO_2$ acts as

Consider the reactions:
$(a)$ $H_3PO_{2(aq)} + 4AgNO_{3(aq)} + 2H_2O_{(l)} \to H_3PO_{4(aq)} + 4Ag_{(s)} + 4HNO_{3(aq)}$
$(b)$ $H_3PO_{2(aq)} + 2CuSO_{4(aq)} + 2H_2O_{(l)} \to H_3PO_{4(aq)} + 2Cu_{(s)} + H_2SO_{4(aq)}$
$(c)$ $C_6H_5CHO_{(l)} + 2[Ag(NH_3)_2]^+_{(aq)} + 3OH^-_{(aq)} \to C_6H_5COO^-_{(aq)} + 2Ag_{(s)} + 4NH_{3(aq)} + 2H_2O_{(l)}$
$(d)$ $C_6H_5CHO_{(l)} + 2Cu^{2+}_{(aq)} + 5OH^-_{(aq)} \to$ No change observed.
What inference do you draw about the behaviour of $Ag^+$ and $Cu^{2+}$ from these reactions?

$Br_2 + 2X^{-}_{aq} \to X_2 + 2Br^{-}_{aq}$ above reaction is correct when $X^{-}$ is :

To an acidic solution of an anion,a few drops of $KMnO_4$ solution are added. Which of the following,if present,will not decolourise the $KMnO_4$ solution?

Identify the reducing agent in the following reaction:
$H_2O_{2(aq)} + ClO_{4(aq)}^{-} \rightarrow ClO_{2(aq)}^{-} + O_{2(g)}$

Starch-iodide paper is used for the test of: