Explanation: The dynatron oscillator operates due to a specific characteristic of a tetrode vacuum tube: **secondary emission from the anode**. In a tetrode, if the anode (plate) voltage is lower than the screen grid voltage, electrons arriving from the cathode can strike the anode with enough energy to knock out additional "secondary" electrons. Because the screen grid is at a higher positive potential, it attracts these secondary electrons away from the anode. This net loss of electrons from the anode means that as the anode voltage *increases* within this region, the anode current actually *decreases*, creating a characteristic known as **negative resistance**. This negative resistance is crucial for sustaining oscillations when the tetrode is connected to a tuned circuit. * A) Positive emission from the anode is incorrect; anodes are typically positive to attract electrons, and "positive emission" isn't a standard term for this mechanism. * C) Cathode saturation refers to the maximum electron emission from the cathode, which is a different operational limit, not the principle behind dynatron oscillation. * D) The dynatron's operation relies on a *negative* resistance characteristic, not a positive one.