The correct option is C Robert Hill
Light reaction is also known as Hill reaction. It is named after the scientist Robert Hill, who first discovered its details. Robert Hill (1937) demonstrated that evolution of O2 occurs during the light reaction. For his experiment, he used isolated chloroplast placed in sunlight in the presence of suitable electron acceptors and absence of CO2. During this process, oxygen was released but sugar was not formed. He concluded that the process by which plants produce oxygen is separate from the process that converts carbon dioxide to sugars.
Melvin Calvin performed experiments which showed that carbon dioxide was reduced to carbohydrates during the light independent reactions of photosynthesis.
Joseph Priestley performed a series of experiments that revealed the essential role of air in the growth of green plants.
Engelmann performed experiments using green algae, Cladophora. He discovered that the rate of photosynthesis (measured by the amount of oxygen released) is high in blue and red light. Hence, he described the first action spectrum of photosynthesis.