In the photosynthesis reaction, specifically in the equation for photosynthesis:
\[ 6 , \text{CO}_2 + 6 , \text{H}_2\text{O} \rightarrow \text{C}6\text{H}{12}\text{O}_6 + 6 , \text{O}_2 \]
there are 6 carbon atoms from the 6 molecules of carbon dioxide (CO₂).
So the correct response is 6.