Answer:
The correct answer is A. carbon fixation.
Step-by-step explanation:
Carboxisomes are the microcompartment which is present in cyanobacteria and some chemotrophic bacteria which contain an enzyme called RuBisCO. This enzyme is responsible for carbon fixation in bacteria and plants.
Carboxysomes contain many polyhedron proteins that contain these enzymes which do carbon fixation in the Calvin cycle. As in carboxysome, the concentration of CO2 around RuBisCO is high so the competition with O2 is decreased or eliminated that favors carbon fixation and production of organic compounds. So the correct answer is A.