Question

which organisms transform nitrogen to a form that is useful to plants?
○ bacteria
○ animals
○ fungi
○ humans

Explanation:

Nitrogen - fixing bacteria convert atmospheric nitrogen ($N_2$) into ammonia ($NH_3$) through nitrogen - fixation. Other bacteria convert ammonia into nitrites ($NO_2^-$) and then into nitrates ($NO_3^-$) through nitrification. Plants can absorb nitrates for their growth. Humans, fungi, and animals do not have the biological mechanisms to directly transform nitrogen in the way described for plant - usable forms.

Answer:

bacteria