Step 1: Understanding the Question:
This question tests knowledge of allelopathy and plant metabolites.
Step 2: Detailed Explanation:
Allelochemicals are secondary plant metabolites.
They are produced by plants to inhibit the growth of other plants.
Primary metabolites (A): Essential for plant growth.
Growth promoters (B): Not allelochemicals.
Plant nutrients (D): Not allelochemicals.
Thus, the correct answer is secondary plant metabolites.
Step 3: Final Answer:
Thus, allelochemicals are secondary plant metabolites, which corresponds to option (C).
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