Explanation: A magnetron requires a strong external magnetic field to operate correctly. This field, perpendicular to the electric field, forces electrons emitted from the cathode into specific circular or helical paths within the tube. This controlled electron movement is essential for energy transfer to the resonant cavities, generating microwaves. Without the magnetic field, the electrons are no longer constrained. The intense electric field between the cathode and anode will accelerate them directly and unimpeded towards the anode. This results in an uncontrolled, extremely high current flow, often called a "runaway current." This excessive current rapidly overheats and destroys the magnetron tube, typically by melting internal components. Therefore, options A, C, and D are incorrect because the primary and immediate consequence is the catastrophic failure of the device due to excessive current, not just distorted output, slight frequency changes, or an insignificant event.