(b) Abscisic acid
Explanation:
During a drought or water stress, plants produce a hormone called abscisic acid (ABA). ABA is responsible for regulating various physiological processes in plants, including responses to water stress.
ABA helps in closing stomata, through which plants can minimise water loss through transpiration and conserve water during periods of drought.
Auxins (option a) are a group of plant hormones involved in growth and development processes.
Ethylene (option c) is a plant hormone involved in various physiological processes, including fruit ripening and senescence.
Cytokinin (option d) are plant hormones that regulate cell division and growth.