The correct option is A Concentration of O2.
The Warburg effect is the decrease of photosynthesis by high oxygen concentrations. Oxygen is a competitive inhibitor of the carbon dioxide fixation by RuBisCO, which initiates photosynthesis.Warburg effect is also due to the process of photorespiration. At high oxygen concentrations, there is a decrease in photosynthesis yield due to wasteful photorespiration process occurring in C3 plants and inhibition of RuBisCO.