Carbon fixation in C3, C4, and CAM plants can be contrasted in terms of the processes involved and the way that special steps are done. C3 is the basic version of oxygen-producing photosynthesis which uses RUBISCO and ribulose bisphosphate to fix carbon dioxide. C4 and CAM both involve C3, done after a special step designed to concentrate carbon dioxide and, thus, make RUBISCO more efficient. C4 uses PEP carboxylase to fix carbon dioxide into a four-carbon organic acid, which is then moved to the special bundle sheath cells for the release of carbon dioxide and C3 photosynthesis (spatial solution). CAM plants use PEP carboxylase at night to take up carbon dioxide, which they release during the day to do C3 photosynthesis (temporal solution).
Work Step by Step
The way to approach this question is to look for the most basic version of photosynthesis (C3), and then contrast the others with it based on how carbon dioxide is fixed. Then find a way to differentiate the other two versions, in this case based on where or when PEP carboxylase is active.