Final answer:
The most likely problem when a three-phase induction motor is humming but not starting is that it is single phasing due to one blown fuse, causing insufficient torque for the motor to start. Therefore, the correct option is A.
Step-by-step explanation:
When a three-phase induction motor starts to hum but not run after an operator pushes the start button, the most probable issue is that the motor is single phasing. Single phasing can occur when one fuse is blown, causing one phase to be disconnected while the other two remain connected. This condition is harmful to the motor as it will try to start but can't develop enough torque and might overheat due to the unbalanced current in the windings. Circuit breakers and fuses are safety devices designed to interrupt excessive currents to prevent thermal hazards and are typically present in electrical systems alongside the familiar three-wire system, which helps guard against thermal and shock hazards. Aspects such as a properly working ground fault interrupter (GFI) and isolation transformer are important for safety and functional operation but are not the issue here.