The correct option is B a(iv), b(i), c(iii), d(ii)
GA3 was one of the first gibberellins discovered. Gibberellins are plant growth promoters that help in germination of seeds and elongation of internodes just before flowering (bolting), stimulation of flowers.
2,4-D belongs to the category of auxins, a type of plant growth promoter. It is used as a weedicide which are chemicals that destroy unwanted plants called weeds. It kills dicots but does not affect the monocots.
Ethylene is a plant growth promoter as well as a plant growth inhibitor. It helps in the ripening of fruits. It also helps accelerate natural detachment (called abscission) of flowers and fruits (thinning of cotton, cherry, walnut).
Cytokinin is a plant growth promoter that has useful effects on cytokinesis. It helps to produce new leaves, chloroplast in leaves, lateral shoot growth, adventitious shoot formation.