The correct option is Gibberellins.
Explanation
Plant growth regulators (PGRs) are organic substances that regulate physiological processes such as growth, differentiation, and development. In agriculture, specific hormones are applied to crops to manipulate vegetative characteristics and enhance yield.
- Gibberellins (Correct): Gibberellins are primarily responsible for internodal elongation. Since sugarcane stores carbohydrates as sugar in its stems, increasing the length of the stem directly increases the storage volume. Spraying sugarcane crops with gibberellins extends the stem length, thereby increasing the yield significantly (often by as much as 20 tonnes per acre).
- Auxins: These hormones are mainly associated with apical dominance (inhibiting lateral bud growth) and root initiation. While they influence cell elongation, they are not the specific agent used for maximizing stem length in sugarcane cultivation.
- Cytokinins: These promote cell division (cytokinesis) and are crucial for shoot initiation and delaying senescence, rather than the specific elongation of internodes.
- Abscisic Acid (ABA): ABA is a growth inhibitor and stress hormone. It promotes seed dormancy and stomatal closure during water stress, which opposes the goal of promoting vegetative growth for yield.
Key Takeaway: Gibberellins promote stem elongation; their application on sugarcane increases the length of the stem and consequently the sugar yield.