The correct option is C Van Niel
Van Niel proposed that oxygen released during photosynthesis comes from water and not from CO2. He could arrive at this based on experiments conducted on purple and green sulphur bacteria. He observed that when these bacteria undergo photosynthesis, oxygen is not released and instead sulphur is released. He argued that if carbon dioxide was the source of oxygen released during photosynthesis, then the byproduct of bacterial photosynthesis should be oxygen but not sulphur. Hence, he concluded that oxygen released in green plants comes from water but not carbon dioxide. In purple and green sulphur bacteria, H2S is the hydrogen donor and hence the oxidation product is sulphur. In green plants, H2O is the hydrogen donor and hence the oxidation product is oxygen.
Julius von Sachs provided evidence for production of glucose when plants grow.
Jan Ingenhousz (1730-1799) showed that sunlight is essential for photosynthesis.
Engelmann did an experiment to describe the action spectrum of photosynthesis.