The reaction that is responsible for the primary fixation of CO2 is catalysed by
1.
RUBP carboxylase
2.
PEP carboxylase
3.
RuBP carboxylase and PEP carboxylase
4.
PGA synthase
In C3 cycle RuBP carboxylase is used to fix atmospheric CO2 whereas, in C4, plants PEP carboxylase is involved in primary CO2, fixation. So, both are used in CO2 fixation but in different cycles.
Other options are incorrect because in (a) and (b) option is giving incomplete information and option (d) PGA synthase is not at all involved in CO2 fixation.
