Answer:
Carbon
Step-by-step explanation:
Photosynthesis is a process carried out by the cells of autotrophic organisms in order to synthesize their own food in form of sugars (glucose). As rightly stated in this question, Carbon dioxide (CO2) is a major ingredient for this process to occur. Inorganic CO2 enters an organism like plankton and is used to make organic molecules.
The organic molecules produced via the photosynthetic process are in form of sugar (glucose) i.e. C6H12O6 whose principal elemental component is CARBON. In other words, the organic molecules contain carbon in their structure, got from the CO2 source.