Concept:Plant hormones regulate growth; gibberellins are specifically known for stimulating stem elongation.Explanation:Gibberellins promote cell elongation and division, especially in stems.They also help in seed germination and flowering.Abscisic acid inhibits growth, ethylene controls ripening, and cytokinins promote cell division, not elongation.Thus, the hormone responsible for cell elongation and growth is gibberellins.Answer:B. Gibberellins