Answer: A
Step-by-step explanation:
Oxidation is the process through which electrons are gained. This indicates that only organisms on the electron side have the potential to be oxidizing agents. c, d, and e are, by definition, eliminated.
We also know that H+ is in the +1 oxidation state. The same is true for 'H' in H2O... it doesn't change oxidation states
Therefore, the answer must be a) nitrate