Which of the following hormones is primarily involved in stress responses in plants?

Abscisic Acid (ABA) is the hormone primarily involved in stress responses in plants. It plays a crucial role in helping plants cope with various stressful conditions, such as drought, salinity, and extreme temperatures. ABA functions by promoting stomatal closure, which reduces water loss during drought stress. This hormone also aids in regulating growth and developmental processes that allow plants to conserve resources under unfavorable conditions.

Moreover, ABA is involved in signaling pathways that activate protective mechanisms in cells, such as the production of stress-responsive genes. By facilitating these adaptive responses, it helps plants survive adverse environmental situations, reinforcing its central role in stress physiology. This hormone's action is distinct from that of ethylene, cytokinins, and auxins, which are involved in different aspects of plant growth and development, such as fruit ripening, cell division, and elongation respectively.