To solve this problem, we need to identify which plant growth regulator increases the length of the stem in sugarcane, thereby increasing the yield. Let's analyze each option:$\newline \bullet$Cytokinin:$\newline$ - Cytokinins are primarily involved in cell division and the delay of leaf senescence. They do not significantly increase stem length.$\newline \bullet$Abscisic acid:$\newline$ - Abscisic acid is mainly involved in stress responses and the inhibition of growth. It does not promote stem elongation.$\newline \bullet$Auxin:$\newline$ - Auxins are involved in cell elongation, but they are more associated with root growth and phototropism rather than significant stem elongation in crops like sugarcane.$\newline \bullet$Gibberellin:$\newline$ - Gibberellins are well-known for promoting stem elongation and increasing the size of fruits and flowers. They are commonly used to increase the length of stems in crops like sugarcane, thereby enhancing yield.$\newline$Therefore, the correct option is Gibberellin, which corresponds to Option 4.