To solve this problem, we need to identify the plant growth regulator that, when sprayed on sugarcane crops, increases the length of the stem, thereby enhancing the yield.$\newline$Let's analyze each option:$\newline \bullet$Gibberellin:$\newline$ - Gibberellins are a group of plant hormones that promote stem elongation, seed germination, and other growth processes.$\newline$ - They are known to increase the length of the stem in sugarcane, which can lead to higher yields.$\newline$ - Therefore, gibberellin is the correct choice for increasing stem length in sugarcane.$\newline \bullet$Ethylene:$\newline$ - Ethylene is a gaseous plant hormone involved in fruit ripening and the response to stress.$\newline$ - It does not primarily promote stem elongation.$\newline \bullet$Abscisic acid:$\newline$ - Abscisic acid is a plant hormone that generally inhibits growth and promotes dormancy.$\newline$ - It is not associated with increasing stem length.$\newline \bullet$Cytokinin:$\newline$ - Cytokinins are involved in cell division and growth, but they do not specifically promote stem elongation like gibberellins.$\newline$Based on the analysis, the plant growth regulator that increases the length of the stem in sugarcane is:$\newline$Option 1: Gibberellin$\newline$Thus, the correct option is Option 1.