The correct answer is B) clogged ventilation ducts. If a DC motor runs hot, the most likely cause is clogged ventilation ducts, which prevent proper airflow and cooling of the motor. Insufficient cooling can cause the motor to overheat and run hot. The other options are incorrect - an open in the series field (A) would cause the motor to stop running, low ambient temperature (C) would not make the motor run hot, and an undercut mica condition (D) would not directly cause overheating.