The correct answer is B) an open start capacitor. If a single-phase capacitor start induction motor fails to start and instead just hums, the most likely problem is an open start capacitor. The start capacitor is essential for providing the initial torque to get the motor rotating. If this capacitor fails and becomes open-circuited, the motor will not be able to develop enough starting torque and will just hum without starting. The other options are incorrect because a blown fuse (A) or a tripped circuit breaker (D) would prevent the motor from even humming, and a shorted centrifugal switch (C) would cause the motor to run continuously without stopping, not just fail to start.