Final answer:
Planktonic organisms at the sea surface that use gases for buoyancy are called neuston. They are part of the diverse aquatic life in the photic zone, with phytoplankton providing food for zooplankton.
Step-by-step explanation:
Planktonic organisms living at the sea surface that utilize buoyancy through gases are known as neuston. These are a variety of aquatic organisms that cannot swim against currents, tides, and waves and they primarily live in the photic zone of water bodies. This region is abundant with life due to the availability of sunlight for photosynthesis, which supports a complex ecosystem. In this ecosystem, phytoplankton, which are bacteria and algae, use sunlight to make food, while zooplankton, which are tiny animals, feed on phytoplankton. The correct answer to the question is C. Neuston.