Living organisms can incorporate b. nitrates and d. ammonia into their metabolism.
- Nitrates can be used by plants in the process of nitrogen assimilation to form amino acids and other important molecules.
- Ammonia can be directly incorporated into organic compounds through various metabolic pathways.
While nitrites can also be used, they are usually an intermediate in the nitrogen cycle. Nitrogen gas (N2) is generally not usable by most organisms directly and needs to be converted into a more reactive form through processes such as nitrogen fixation.
So, if you need to choose one answer from the options given, I would suggest b. nitrates or d. ammonia as the most appropriate.