The correct option is B a-(iv), b-(i), c-(iii), d-(ii)
2,4-D (auxin) is widely used to kill dicotyledonous weeds and hence it is a weedicide.
GA3 (gibberellic acid) induces bolting which means elongation of internodal region just prior to flowering.
Thinning of cotton means detachment of cotton balls (abscission of flower). It is hastened by ethylene.
Cytokinin promotes lateral growth of shoot by overcome the apical dominance.