The correct option is A The primary fixation of CO2 is mediated via PEP carboxylase.
CO2 concentration of the atmosphere is 360 ppm. It is a limiting factor for C3 as the available CO2 concentration is lower than the optimum for photosynthesis. Increase in its concentration upto 0.05% increases the rate of photosynthesis in most C3 plants. When CO2 concentration is reduced, there comes a point at which illuminated plant parts stop absorbing CO2 from their environment. It is called as CO2 compensation point or threshold value. The C4 plants have this compensation value low as they are more efficient in CO2 fixation through PEP- Carboxylase. The optimum CO2 concentration for C4 plants is 360 ppm. But the atmospheric conc of CO2 is increasing. If the trend of CO2 increase continues then that would be harmful for C4 plants.