The correct option is A Gibberellin
Gibberellin is a plant growth promoter that can be sprayed on the crops to get a better yield. The most characteristic effects of GA on shoot growth are increased inter‐node extension, increased leaf‐growth and enhanced apical dominance. So this effect it can can increase the length of the stem thus increasing the yield as much as 20 tonnes per acre.
Cytokinin is a plant growth promoter whose main function is the induction of cell division and controlling apical dominance. Apical dominance is a phenomenon in which the growth of the main central stem of the plant is more as compared to the other stems. So they do not have a noticeable impact on the productivity of sugarcane crops.
Ethylene is a plant growth promoter as well as a plant growth inhibitor. It helps in the ripening of fruits. They also do not have a noticeable impact on the productivity of sugarcane crops.
Abscisic acid is a plant growth inhibitor that plays an important role in seed development, maturation and seed dormancy. Seed dormancy is the condition of seeds that are incapable of germinating even when the conditions suitable for seedling growth are supplied. They also do not have a noticeable impact on the productivity of sugarcane crops.