Explanation: Gunn diode oscillators take advantage of the **bulk-effect**, also known as the Gunn effect, which occurs in certain semiconductor materials like gallium arsenide (GaAs). When a sufficient electric field is applied across the material, electrons transfer from a low-energy, high-mobility conduction band valley to a higher-energy, low-mobility valley. This transfer causes the average electron velocity, and thus the current, to decrease as the voltage increases, leading to a region of **negative resistance**. This negative resistance is the key property that allows the Gunn diode to generate microwave oscillations when incorporated into a resonant circuit. The bulk-effect is the underlying physical phenomenon that *causes* the negative resistance. Therefore, both concepts are fundamental to the operation of a Gunn diode oscillator. Option A is incomplete because negative resistance is the *result* of the bulk-effect. Option B, avalanche transit time, describes the principle of operation for IMPATT diodes, not Gunn diodes. Option C is incomplete because while the bulk-effect causes the negative resistance, it is the negative resistance itself that allows for oscillation.