Key Concept: Allelopathy is a chemical, not physical, interaction between plant species.
Why it's correct: One species releases biochemicals (allelochemicals) that inhibit the growth of nearby species — e.g., black walnut's juglone suppressing neighboring plants.
Why others fail: Symbiosis and mutualism describe mutually BENEFICIAL relationships, the opposite of allelopathy's inhibitory effect; competition for light is a physical resource conflict, not a chemical one.