Deforestation kills many organisms that produce oxygen. What oxygen-producing organisms would remain if all the forests were destroyed? A. Rocks B. Humans C. Plankton D. Animals

It would be C. Plankton.

The correct answer for the given question above would be option C. Since deforestation is killing organisms that produce oxygen, which are the plants and trees, the only oxygen-producing organisms that would remain if all the forest were destroyed are the planktons. The planktons produce oxygen, especially in the ocean. The oxygen is the byproduct of the process of photosynthesis of these planktons.