The best answer is C. - soil
The nitrogen used by most plants is derived from soil.
Nitrogen is an important element. It is needed by organisms to produce amino acids
Nitrogen is made usable for plants by bacteria which change nitrogen to ammonium.
Ammonium gets changed to nitrates by bacteria in a process called nitrification, which takes place in soil.
Nitrates is the form that plants can absorb. They absorb it through their roots and use it to make amino acids, nucleic acid and chlorophyll.