To solve this problem, we need to identify which hormone promotes internode or petiole elongation in deep water rice. Let's analyze each option:$\newline \bullet$Option 1: $GA_3 \newline$ - Gibberellic acid (GA_3) is known to promote stem elongation and growth in many plants. However, in the specific case of deep water rice, it is not the primary hormone responsible for internode elongation.$\newline \bullet$Option 2: Kinetin$\newline$ - Kinetin is a type of cytokinin, which primarily promotes cell division and delays senescence. It is not directly involved in internode elongation in deep water rice.$\newline \bullet$Option 3: Ethylene$\newline$ - Ethylene is a plant hormone that plays a crucial role in the elongation of internodes in deep water rice. It helps the plant adapt to submerged conditions by promoting rapid elongation, allowing the plant to reach the water surface.$\newline \bullet$Option 4: 2, 4–D$\newline$ - 2, 4-D is a synthetic auxin used primarily as a herbicide. It is not involved in promoting internode elongation in deep water rice.$\newline$Based on the analysis, the correct hormone that promotes internode/petiole elongation in deep water rice is Ethylene.$\newline$Therefore, the correct option is Option 3.