Oxygen enters the ocean only at the surface, from and a. photosynthesis, the atmosphere b. volcanic eruptions, meteorites c. photosynthesis, respiration d. respiration, photosynthesis e. respiration, rainfall

Oxygen enters the ocean from photosynthesis and the atmosphere.

The atmosphere contains oxygen molecules that when those molecules enter in contact with the ocean water, they get dissolved in the water.
The other way is through photosynthesis. Just like in the land, there are photosynthetically active plants and bacteria in the ocean, that converts carbon dioxide and water, and releases oxygen.