a. In `SO_(2)`, oxidation number of `S` is `+4`. In principle `S` can have a minimum oxidation number of `-2` and maximum of `+6`. Therefore, `S` in `SO_(2)` can either decrease or increase its oxidation number and hence can act both as an oxidising as well as a reducing agent.
b. In `H_(2)O`, the oxidation number of `O` is `-1`. In principle, `O` can can have a minimum oxidation number of `-2` and maximum of zero `(+2 " is possible only with " OF_(2))`. Therefore, `O` in `H_(2)O_(2)` can either decrease its oxidation number from `-1` to `-2` or can increase its oxidation number from `-1` to `0`. Therefore, `H_(2)O_(2)` acts as an oxidising as well as a reducing agent.
c. In `O_(3)` the oxidation number of `O` is zero. It can only decrease its oxidation number from from `0` to `-1` or `-2`, but cannot increase to `+2`. Therefore, `O_(3)` acts only as an oxidant.
d. In `HNO_(3)`, oxidation number of `N` is `+5` which is maximum. Therefore, it can only decrease its oxidation number and hence it acts as an oxidant only.
