The correct answer is D) a steam trap is hung open. Excessively hot water returning to an atmospheric drain tank indicates that a steam trap is hung open, allowing steam to pass through instead of condensing. Steam traps are designed to remove condensate from the system while preventing the passage of live steam. If a steam trap is malfunctioning and stuck open, it will allow hot steam to enter the condensate return system, resulting in excessively hot water returning to the drain tank. The other options are incorrect because they do not directly explain the cause of the excessively hot water. A loss of circulating water (A) or a ruptured heating coil (C) would not necessarily result in hot water returning to the drain tank. Additionally, a condensate recirculating valve being open (B) would not be the primary cause of the issue.