The answer is nitrogen. Plants cannot use atmospheric nitrogen directly. Nitrogen from the air must be converted into the different form so plants can absorb it. Thus plants need nitrogen-fixing bacteria to convert atmospheric nitrogen into ammonium, which is available for plants to absorb it.
Plants rely on nitrogen-fixing bacteria to convert atmospheric nitrogen (N₂) into usable forms such as ammonia. These bacteria are essential for plant growth as nitrogen is vital for synthesizing proteins and nucleic acids. Thus, the correct answer is C. Nitrogen.
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