Explanation: A motor generator failing to start indicates a problem in the electrical circuit supplying the motor or within the motor itself. * **A) Blown fuse or tripped circuit breaker:** These protective devices are designed to interrupt the circuit if an overcurrent or fault occurs. If they are open, no power reaches the motor, preventing it from starting. This is a common and fundamental electrical fault. * **B) Open armature or motor-field rheostat:** An "open" circuit means there's a break in the electrical path. If the armature winding (the rotating coil) has an open circuit, current cannot flow through it to create the necessary magnetic field and torque. Similarly, an open motor-field circuit (perhaps through a defective rheostat) means the stationary magnetic field won't be established, which is essential for the motor's operation. * **C) Broken connections, defective brushes:** Broken connections anywhere in the motor's power supply or internal windings will interrupt current flow. Defective brushes (worn, broken, or not making proper contact) prevent current from reaching the armature windings via the commutator, rendering the motor inoperable. Since each of these conditions (A, B, and C) describes a distinct and valid reason why an electric motor would fail to start, "All of the above" is the correct and comprehensive answer.