C  

Cyanide binds to the heme-a3/CuB center of cytochrome-c oxidase (complex IV), arresting electron flow and preventing NADH from being re-oxidised to NAD⁺. Without regenerated NAD⁺, the dehydrogenase steps of the citric acid cycle stop, so the cycle is functionally “blocked” at the level of NAD availability (option C) rather than at individual enzymes such as aconitase or citrate synthase [Jensen 1984, PMID 6098268].