Final answer:
The most productive region of the ocean is the photic zone, which ranges from the surface to about 200 meters depth, due to the availability of light for photosynthesis.
Step-by-step explanation:
The question pertains to the productivity of various ocean zones. Productivity in this context relates to the biological production of organic compounds from inorganic substances, primarily through photosynthesis. The photic zone, which extends from the surface to about 200 meters, is where sunlight penetrates the water, enabling photosynthesis. This zone contains phytoplankton, which form the base of the marine food web and are responsible for about half of the Earth's photosynthesis.
Below the photic zone lies the aphotic zone, where light does not reach, making photosynthesis impossible. Consequently, productivity in the aphotic zone is far lower because it relies on materials sinking from the surface rather than direct biological production. The abyssal zone and deeper regions like the Challenger Deep are also part of the aphotic zone, where life exists but is sparse and adapted to extreme conditions, with productivity primarily driven by chemosynthesis rather than photosynthesis.
In summary, the most productive region of the oceanic zones is the surface or the photic zone, where light is available for photosynthesis, and where the highest concentration of marine life can be found, including phytoplankton that play a crucial role in carbon fixation and supporting marine food webs.