Question

which step shows nitrogen fixation? conversion of atmospheric n₂ into ammonium uptake of nitrates by plants conversion of nitrates to nitrogen gas conversion of nitrogen gas into nitrates

Explanation:

Nitrogen fixation is the process of converting atmospheric nitrogen gas ($N_2$) into ammonia or other nitrogen - containing compounds that can be used by plants. In the nitrogen cycle diagram, the step labeled "Nitrogen fixation" shows the conversion of atmospheric $N_2$ into a usable form.

Answer:

Conversion of atmospheric $N_2$ into ammonium