The enzyme ribulose- 1, 5- bisphosphate carboxylase/oxygenase is most commonly known by the name RuBisCO and is used in the Calvin cycle to catalyze the first major step of carbon fixation. RuBisCO is thought to be the most abundant protein in the world since it is present in every plant that undergoes photosynthesis and molecular synthesis through the Calvin cycle.

So, the correct answer is option C