To solve this problem, we need to identify which growth regulator is known as the stress hormone. Let's analyze each option:$\newline \bullet$Abscisic acid:$\newline$ - Abscisic acid (ABA) is a plant hormone that plays a crucial role in stress responses, particularly in response to drought and other environmental stresses.$\newline$ - It helps in closing stomata to reduce water loss and induces dormancy in seeds and buds.$\newline$ - Due to its role in stress responses, it is commonly referred to as the stress hormone.$\newline \bullet$Ethylene:$\newline$ - Ethylene is a gaseous plant hormone involved in fruit ripening, leaf abscission, and response to mechanical stress.$\newline$ - While it plays a role in stress responses, it is not primarily known as the stress hormone.$\newline \bullet$GA₃ (Gibberellic acid):$\newline$ - Gibberellic acid is involved in promoting growth and development processes such as seed germination, stem elongation, and flowering.$\newline$ - It is not associated with stress responses.$\newline \bullet$Indole acetic acid (IAA):$\newline$ - Indole acetic acid is a type of auxin, a plant hormone that regulates growth and development, including cell elongation and differentiation.$\newline$ - It is not primarily involved in stress responses.$\newline$Based on the analysis, abscisic acid is the growth regulator known as the stress hormone.$\newline$Therefore, the correct option is Option 1: Abscisic acid.