Which one among the following is not a plant hormone?

Thyroxine is not a plant hormone; it is an animal hormone produced by the thyroid gland in vertebrates, including humans, and regulates metabolism, growth, and development. The five main classes of plant hormones (phytohormones) are: Auxins (promote cell elongation and apical dominance), Gibberellins (promote stem elongation and seed germination), Cytokinins (promote cell division), Abscisic acid (induces dormancy and stomatal closure during stress), and Ethylene (promotes fruit ripening and senescence). Therefore, among the given options, thyroxine is the only one that is not a plant hormone.