Plants use carbon dioxide (CO₂) for photosynthesis, not respiration. Respiration is the process where plants break down sugars to release energy, producing CO₂ as a byproduct. However, the carbon in the CO₂ released comes from the organic compounds within the plant, which were originally built using the most common, stable carbon isotope: Carbon-12 (C-12). While plants can incorporate other isotopes like C-13 or C-14, the primary and most abundant carbon used in biological processes is C-12.

Final Answer: C - 12