Final answer:
Abscisic acid (ABA) primarily functions to inhibit root growth, as it acts in response to stressful conditions such as drought to help conserve plant resources by closing stomata and maintaining dormancy.
Step-by-step explanation:
The function of abscisic acid (ABA) in roots is predominantly related to responding to stressful environmental conditions such as dehydration. While ABA was once thought to be associated with the process of abscissoin, it is now known that its role is different. Specifically, ABA contributes to closing the stomata to prevent water loss during times of drought, and it plays a role in maintaining dormancy in seeds and other organs. Therefore, when considering the options provided, the best fit is option 2) Inhibits root growth, as ABA is responsible for counteracting growth-promoting effects under stressful conditions to help the plant conserve its resources.