Question

which process converts nitrogen gas into ammonia? ammonification nitrification denitrification nitrogen fixation

Explanation:

Ammonification is the conversion of organic - nitrogen compounds to ammonia. Nitrification is the oxidation of ammonia to nitrites and then nitrates. Denitrification is the reduction of nitrates to nitrogen gas. Nitrogen fixation is the process of converting atmospheric nitrogen gas ($N_2$) into ammonia ($NH_3$).

Answer:

Nitrogen fixation