(a) Promote cell division.
Cytokinin is a plant hormone that promotes cell division (cytokinesis) in plants.
The question tests knowledge of plant hormones from Chapter 6. Cytokinin specifically promotes cell division and is used in tissue culture to stimulate rapid cell division (callus formation). The other options describe functions of Abscisic acid (wilting/stomatal closure) and Auxin/Gibberellin (stem growth). Examiners expect direct identification of the correct option with a brief supporting reason.