The letter that marks the location where carbon dioxide is produced during cellular respiration is letter 'X' (mitochondria).
Where carbon dioxide is produced
During the electron transport chain, NADH and FADH2 generated in the citric acid cycle transfer electrons to the electron transport chain in the inner mitochondrial membrane. This process ultimately generates more ATP.
Therefore, most of the CO2 produced during cellular respiration comes from the citric acid cycle within the mitochondria, while a small amount is released in the pyruvate dehydrogenase complex step.