The question asks for a plant growth regulator (PGR) that functions as a herbicide. We evaluate the given options: Gibberellin (growth promoter), 2, 4-D (synthetic auxin), Kinetin (cytokinin), and Abscisic acid (growth inhibitor).
2, 4-D stands for 2, 4-dichlorophenoxyacetic acid. It is a synthetic auxin. In agriculture, it is widely used as a selective herbicide to kill broad-leaved weeds without affecting monocot crops like wheat or maize.
Gibberellins are used to increase fruit size and delay senescence. Kinetin is used for tissue culture and cell division. Abscisic acid is primarily involved in stress responses and seed dormancy. None of these are primarily classified as herbicides.
Final Answer: 2, 4-D
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