"The saturation point for C ₃  is obtained at higher concentrations as compared to C ₄ . Therefore, CO ₂  concentration is more of a limiting factor for C ₃  plants." Can you please explain this statement?

Saturation point of CO₂ is defined as the concentration of carbon dioxide beyond which no change is observed in the rate of photosynthesis. C₄ plants have a highly efficient way of CO₂ assimilation. There are two layers of cells; Mesophyll cells which take up carbon dioxide from  atmosphere and transfer it to bundle sheath cell which fix the carbon via calvin cycle. So, CO₂ is used very efficiently and at lower levels of CO₂ the plants get saturated. But this is not the case with C₃ plants hence they get saturated at higher CO₂ concentrations.