Which of the following acts as an oxidising as well as reducing...

As in $HN{O}_{2}$ , $\text{N}$ is in $\text{+3}$ oxidation state but nitrogen can have oxidation states from $- \text{3}$ to $+ 5$ so it is in between the maximum and minimum oxidation state hence it can either get reduced or oxidized, depending on the conditions. It means it can act both as an oxidizing and a reducing agent. It can work both as oxidising as well as reducing agent.
(I) As an oxidising agent:
$HN{O}_{2} + 2H^{+} + e^{-} \to NO + {H}_{2}O$
(II) As reducing agent:
$HN{O}_{2} + {H}_{2}O \to N{O}_{3}^{-} + 3H^{+} + 2e^{-}$