Answer
See answer below.
Work Step by Step
$HNO_3$: oxidizing agent in a reaction where $H_2$ is released because the oxidation number of H decreases (+1 to 0) or in a reaction where $NO_3^-$ is converted to $NO_2^-$ because the oxidation number of N decreases (+5 to +3).
$Na$: reducing agent, the ionization to $Na^+$ would increase the oxidation number of Na (0 to +1).
$Cl_2$: oxidizing agent, the ionization to $Cl^-$ or a covalent bond where Cl is the most electronegative would decrease the oxidation number of Cl (0 to -1).
It can be a reducing agent in a reaction where a $ClF$ is a product.
$O_2$: oxidizing agent, the most common oxidation number of O is -2, or even -1 in peroxides, so its oxidation number would decrease in most reactions.
It can be a reducing agent if a product of the reaction is $OF_2$
$KMnO_4$: oxidizing agent, common oxidation numbers for Mn are +2 or +4 and 0 in the metal, so it would decrease from +7 in $KMnO_4$.
