B6 atoms of carbon C
I would think this is the answer, because one can't just delete or add atoms; otherwise, the equation would be unbalanced. This also abides with the Law of Conservation of Mass.
Plus, I also came to that conclusion because if we look at the net equation of photosynthesis:
\(6C O_{2} + 6H_{2}O\) ––> \(C_{6}H_{12}O_{6} + 6O_{2}\)
The number of Carbon atoms is 6 on both the reagent's side and the product's side.
