Indolebutyric acid (IBA) is primarily used for which purpose in horticulture?

Indolebutyric acid (IBA) is primarily used in horticulture for promoting the rooting of cuttings. This plant hormone encourages the formation of roots when applied to the cut ends of stem or leaf cuttings, which is essential in plant propagation. By stimulating root growth, IBA helps ensure that the cuttings can develop a stable plant and thrive once planted in soil. This is particularly important in a variety of horticultural applications, from commercial plant production to home gardening, as successful rooting increases the chances of survival and establishment of new plants.

The options of promoting flowering, enhancing color in blooms, and preventing disease do not align with IBA's primary function. While some hormones might play roles in other plant processes such as flowering or color enhancement, IBA specifically targets root development.