Correct Option: C  [ ammonia ]
Remark:
Explanation: Plants absorb nitrogen from the soil mainly in the form of nitrates (NO₃⁻). Inside the plant, nitrates are reduced to nitrites (NO₂⁻) and then to ammonia (NH₃) through the action of specific reductase enzymes. The ammonia thus formed is then used to synthesize amino acids, proteins, nucleic acids, and other nitrogen-containing compounds necessary for plant growth.
Simplified Pathway:
NO₃⁻ → NO₂⁻ → NH₃ → Amino acids → Proteins