Plant hormones, also known as phytohormones, play essential roles in regulating various aspects of plant growth and development. Among the following options, the plant hormone is “auxin.” Auxins are a class of plant hormones that control processes such as cell elongation, apical dominance, and root formation. The other options, cytokinins, gibberellins, and abscisic acid, are also plant hormones that have different functions in plant physiology.