33. Photosynthesis in C4 plants is relatively less limited by atmospheric CO2 levels because
(a) effective pumping of CO2 into bundle sheath cells
(b) RuBisCo in C4 plants has higher affinity for CO2 
(c) four carbon acids are the primary initial CO2 fixation products
(d) the primary fixation of CO2 is mediated via PEP carboxylase.

Dear student,
The correct answer is (d). The photosynthesis in C4 plants is relatively less limited as compared to other plants like C3 and CAM by atmospheric CO2 levels because the primary fixation of CO2 is mediated with the help of PEP carboxylase in C4 plants. PEP carboxylase plays an important role in binding of carbon dioxide with PEP   in the form of bicarbonate to form oxaloacetate in the mesophyll tissue.Thus, C4 plants use PEP carboxylase to capture more carbon dioxide in the mesophyll cells.

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