To solve this problem, we need to identify which phytohormone is responsible for increasing the yield of cucumbers by promoting the production of female flowers.$\newline$Let's analyze each option:$\newline \bullet$ABA (Abscisic Acid):$\newline$ - ABA is primarily known for its role in stress responses and seed dormancy.$\newline$ - It does not promote the production of female flowers.$\newline \bullet$Gibberellin:$\newline$ - Gibberellins are involved in promoting stem elongation, seed germination, and flowering.$\newline$ - However, they are not specifically known for increasing female flower production in cucumbers.$\newline \bullet$Ethylene:$\newline$ - Ethylene is a gaseous hormone that influences fruit ripening and flower development.$\newline$ - It is known to promote the formation of female flowers in cucumbers, leading to increased yield.$\newline \bullet$Cytokinin:$\newline$ - Cytokinins are involved in cell division and growth, as well as delaying leaf senescence.$\newline$ - They do not specifically promote female flower production in cucumbers.$\newline$Based on this analysis, the phytohormone responsible for increasing cucumber yield by promoting female flowers is Ethylene.$\newline$Therefore, the correct option is Option 3: Ethylene.