The correct option is A Julius von Sachs
Julius von Sachs found that starch is the primary visible result of photosynthesis. He experimented on etiolated plants. He took a plant that was kept in the dark for a few days and moved it to sunlight. Then after sometime, he moved it to the dark room. He repeated the same process again (moving to sunlit room). When the plant was kept in sunlight, starch granules were synthesised. When the plant was moved back to the dark room, the starch stored in the plants slowly disappeared. When the plant was exposed to sunlight again, starch was found to be produced. It eventually got accumulated in the plant. He finally concluded that production and storage of starch depend on plant’s exposure to sunlight. And also that starch production was necessary for plant growth.
Further experiments by him also revealed that it is the green substances (now called chlorophyll) in the plants are located inside special structures (now called chloroplasts) within plant cells.
Through a series of experiments, Priestley made a tremendously significant observation: A fire would douse itself and a mouse would choke out when set in the sealed container. He put a green plant in the container and presented it to daylight which allowed the fire to stay lit and the mouse to stay alive. Later, he discovered that plants discharge oxygen into the air during photososynthesis.
Cornelius van Niel conducted his studies on purple & green bacteria. During photosynthesis, hydrogen gets released from an oxidisable compound which reduces carbon dixide and carbohydrates are formed as a result.
Theodore de Saussure was instrumental in showing that carbon dioxide was consumed by the plants during photosynthesis.