Final answer:
Horticulture is most often conducted in River Valleys, where conditions are favorable for intensive cultivation due to fertile soils and available water.
Step-by-step explanation:
Horticulture is most often conducted in River Valleys (B). River valleys frequently have fertile soils and a consistent water supply that are ideal for the cultivation of crops and other horticultural activities. Unlike coastal regions where plantation agriculture dominates or rainforests where slash-and-burn horticulture is practiced in a more extensive form, river valleys offer the essential conditions for intensive forms of plant cultivation which include the repetitive use of land and the cultivation of a wide variety of crops.