The correct answer to the question is Option B, Gibberellin. Gibberellins (GAs) are a group of plant hormones that play a crucial role in regulating various aspects of plant growth and development. One of the notable effects of gibberellins is their ability to promote stem elongation by stimulating cell division and elongation.

In the case of sugarcane, a crop that is valued for its stem (from which sucrose is extracted), applying gibberellin can increase the length of the stem, thereby potentially increasing the yield of the crop. This increase in stem length due to gibberellin application is primarily because gibberellins overcome the inhibitory effects of other hormones that suppress growth; they stimulate the cells in the stems to grow larger and divide more frequently.

While Auxin (Option A) and Cytokinins (Option C) also influence plant growth and have their specific uses, such as rooting and promoting cell division, respectively, they do not primarily target stem elongation to the extent gibberellins do. Abscisic acid (Option D) is generally involved in stress responses and the inhibition of growth, making it unsuitable for promoting stem elongation.

Therefore, to specifically enhance stem length and yield in sugarcane, gibberellins are the most effective plant growth regulators among the options given.