Judging by the oxidation state of the non-hydrogen element, which of the following can act ONLY as a reducing agent?

S in $\mathrm{H}{}_2\mathrm{S}$ is at its lowest oxidation state (-2) so it can only lose electrons. N in $\mathrm{HNO}{}_3$ is at its highest (+5), S in $\mathrm{SO}{}_2$ is +4 (intermediate), and O in $\mathrm{H}{}_2\mathrm{O}{}_2$ is -1 (intermediate). Hence $\mathrm{H}{}_2\mathrm{S}$.