Explanation: A series DC motor's field winding is in series with its armature. This means the magnetic field strength (flux) is directly proportional to the armature current, which itself is determined by the mechanical load. When a series DC motor operates without a mechanical load, the armature current drops significantly. This causes a drastic reduction in the motor's magnetic field flux. Motor speed is inversely proportional to the field flux. With a very weak field, the motor speed can theoretically become infinite, leading to a dangerous "runaway" condition where extreme centrifugal forces can physically destroy the motor windings or components. Operating on AC (B) or under varying load (C) would cause different operational issues, inefficiencies, or normal speed fluctuations, but not the specific destructive runaway failure mode caused by a lack of load on a DC series motor.