Which of the following is the strongest reducing agent?

$Cu^{2+}$ does not form $CuI_2$. This is because $Cu^{2+}$

When acidified $K_2Cr_2O_7$ solution is added to $Sn^{2+}$ salt,then $Sn^{2+}$ changes to . . . . . . .

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

Which one of the following statements is true for $HNO_2$?

Sulphur dioxide and hydrogen peroxide can act as both oxidizing and reducing agents in their reactions,while ozone and nitric acid act only as oxidants. Why?