Which one of the plant growth regulators would you use if you are asked to:
$(a)$ 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.

(N/A) Induce rooting in a twig: Auxins (e.g.,$IBA$,$NAA$ are used for rooting in stem cuttings).
$(b)$ Quickly ripen a fruit: Ethylene (it is a gaseous hormone that promotes fruit ripening).
$(c)$ Delay leaf senescence: Cytokinins (they promote cell division and delay the aging process in leaves).
$(d)$ Induce growth in axillary buds: Cytokinins (they help overcome apical dominance and promote lateral bud growth).
$(e)$ 'Bolt' a rosette plant: Gibberellic acid (it induces internode elongation just prior to flowering in rosette plants like cabbage).
$(f)$ Induce immediate stomatal closure in leaves: Abscisic acid (it acts as a stress hormone and triggers stomatal closure during water stress).