Which of the following,after reacting with $KI$,does not liberate iodine?

Which acts as an oxidising agent in the following reaction?
$2H_2O_{(l)} + 2Na_{(s)} \to 2NaOH_{(aq)} + H_{2_{(g)}}$

In the reaction $HAsO_2 + Sn^{2+} \to As + Sn^{4+} + H_2O$,the oxidizing agent is:

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

Starch iodide paper is used to test for the presence of .........

The strongest reducing agent among the given options is: