(a) induce rooting in a twig

(b) quickly ripen a fruit

(c) delay leaf senescence

(d) induce growth in axillary buds

(e) ‘bolt’ a rosette plant

(f) induce immediate stomatal closure in leaves

Solution:

Listed below are the plant growth regulators for the corresponding events:

(a) induce rooting in a twig – Auxins

(b) quickly ripen a fruit – Ethylene

(c) delay leaf senescence – Cytokinins

(d) induce growth in axillary buds – Cytokinins

(e) ‘bolt’ a rosette plant – Gibberellins

(f) induce immediate stomatal closure in leaves – Abscisic acid