If an unloaded DC compound motor's shunt field were weakened by excessive rheostat resistance or by an open circuit, how would the motor respond?

Correct Answer: Option D — over speed due to reduced CEMF

Weakening or losing the shunt field in an unloaded compound motor drastically reduces magnetic flux. Counter-electromotive force (CEMF) drops precipitously, causing the machine to dangerously overspeed and potentially experience catastrophic mechanical failure from centrifugal forces.