Final answer:
Coastal areas are most likely to support highly productive ecosystems due to upwelling, because this process brings nutrient-rich waters to the surface fostering abundant marine life.
Step-by-step explanation:
The regions of the ocean most likely to support highly productive ecosystems due to upwelling are coastal areas. Upwelling is a process where deep, cold, and nutrient-rich water rises to the surface, usually along coastlines, and supports the growth of phytoplankton.
As a foundational component of the marine food web, these phytoplankton enable high productivity, supporting a diverse array of marine life, including fish, invertebrates, and marine mammals. Compared to the minimal light penetration in the deep-sea trenches, the open ocean's vast expanse with varying conditions, and the more isolated coral reefs, coastal areas with upwelling are unique in consistently providing the copious amounts of nutrients needed for thriving marine ecosystems.