In the process of photosynthesis, where does the carbon used to synthesize glucose primarily come from?

Analyze the options provided: (1) Carbon dioxide ($\mathrm{CO}{2}_{}$) in the atmosphere, (2) Organic molecules in the soil, (3) Minerals taken up from the soil, and (4) Water ($H{2}_{}O$) absorbed by roots.

Eliminate incorrect options: (a) Organic molecules in the soil are not directly used in photosynthesis. (b) Minerals from the soil do not provide carbon for glucose synthesis. (c) Water ($H{2}_{}O$) provides hydrogen and oxygen but not carbon.