The correct answer is C) a steam trap is hung open. Excessively hot water returning to an atmospheric drain tank indicates that a steam trap has failed in the open position. Steam traps are used to automatically remove condensate (hot water) from a steam system while preventing the escape of live steam. If a steam trap is hung open, it will allow hot condensate to continuously flow back to the atmospheric drain tank, rather than being properly drained from the system. The other options are incorrect because they do not directly explain the presence of excessively hot water in the drain tank. A loss of circulating water (A) or a ruptured heating coil (B) may lead to other issues, but would not necessarily result in hot water draining back to the tank. Option D, the condensate recirculating valve being open, is not the primary cause of the hot water in this scenario.