The correct answer is C) Tight bearings/frozen shaft. If a DC motor hums but does not run when energized, the most likely cause is tight bearings or a frozen shaft. This condition prevents the motor from turning, even though it is receiving power. The humming sound indicates that the motor windings are being energized, but the mechanical issues are preventing the rotor from rotating. The other options are less likely in this scenario. A) No load on the motor would not cause the motor to hum, and B) a blown fuse or tripped circuit breaker would prevent the motor from receiving power and being energized at all. D) A tripped overload could also prevent the motor from running, but this would typically be accompanied by the motor not humming.