The correct option is B Carbon dioxide
CO2 is the major limiting factor for photosynthesis. The concentration of CO2 in the atmosphere lies between 0.03 %- 0.04%. An increase in the concentration of CO2 up to 0.05% in the atmosphere can cause an increase in CO2 fixation rates. Beyond this, CO2 is accumulated in the plants and results in slowdown of the process. At low light intensities, C3 plants do not respond to high CO2 conditions as in such cases light becomes the limiting factor. At high light intensities, they show an increase in the rates of photosynthesis when exposed to high CO2 conditions. Higher productivity has been observed in some C3 plants like tomatoes when they are grown in a CO2 enriched atmosphere.