Final answer:
Nitrogen must be fixed by bacteria before it can be used by plants and animals. This process involves bacteria converting nitrogen gas from the atmosphere into compounds plants can absorb.
Step-by-step explanation:
Before plants and animals can use nitrogen, it must be transformed into a molecule that living organisms can metabolize. This essential process is known as nitrogen fixation, carried out by specific bacteria. These bacteria are capable of converting nitrogen gas (N₂) from the atmosphere into organic compounds such as ammonia, which can then be further processed into nitrates and nitrites (nitrification). Plants are then able to absorb these nitrates and nitrites through their roots and use them to synthesize necessary proteins and nucleic acids.
Therefore, the correct answer to the student's question is b. Nitrogen must be fixed by bacteria. Other steps in the nitrogen cycle include ammonification, where organic nitrogenous matter is converted into ammonium (NH₄+), and denitrification, where soil bacteria convert nitrates (NO₃⁻) into nitrogen gas (N₂), returning it to the atmosphere.