Van Neil's hypothesis about the production of oxygen during photosynthesis was based on the study and investigations of:

Explanation

Van Niel's hypothesis was formulated from his work with photosynthetic bacteria that utilized hydrogen sulfide (H₂S) instead of water, releasing sulfur instead of oxygen. This led him to propose that plants split water (H₂O) to release oxygen during photosynthesis. His hypothesis was later confirmed by experiments using heavy isotopes of oxygen (O18). The other options-algae, protonema, and cyanobacteria-were not the focus of Van Niel's original research on this specific hypothesis.