Spraying sugarcane crop with which of the following plant growth regulators, increases the length of stem, thus, increasing the yield?

The correct option is Gibberellin.

Step-by-step Explanation:

1. Sugarcane stores carbohydrates (sugar) in its stems. Therefore, the yield of a sugarcane crop is directly related to the length and biomass of its stem.
2. Gibberellins are a group of plant hormones (plant growth regulators) that play a critical role in promoting stem elongation, internode extension, and overall plant height.
3. When sugarcane crops are sprayed with gibberellins, it stimulates rapid cell division and cell elongation in the stem internodes.
4. This elongation increases the length of the stem significantly, which directly increases the yield of the sugarcane crop (by up to 20 tonnes per acre).
5. Other plant growth regulators have different primary functions: Auxins are mainly involved in apical dominance and root initiation; Cytokinins promote cell division and delay senescence; and Abscisic acid acts as a growth inhibitor and stress hormone.