Question

which statement best describes nitrogen fixation?
a process in which bacteria convert ammonium into nitrate and nitrite
a process in which bacteria convert organic forms of nitrogen into ammonia or ammonium
a process in which nitrate is converted to nitrogen gas
a process in which bacteria break nitrogen gas down into a usable form for plants

Explanation:

Nitrogen fixation is the process by which bacteria convert atmospheric nitrogen gas ($N_2$), which is not directly usable by most plants, into ammonia ($NH_3$) or other nitrogen - containing compounds that plants can use. The first option describes nitrification, the second describes ammonification, and the third describes denitrification.

Answer:

a process in which bacteria break nitrogen gas down into a usable form for plants