Explanation: An astable multivibrator is a free-running oscillator that continuously alternates between two temporary, unstable states without requiring an external trigger. It has no stable operating points, meaning it won't stay in one state indefinitely. This characteristic allows it to generate a continuous square-wave output, fundamental in timing and clock circuits in radio equipment. Option D accurately describes this behavior: "A circuit that alternates between two unstable states." Option A describes a bistable multivibrator (like a flip-flop), which has two stable states and remains in one until externally triggered. Option B describes a monostable multivibrator (or "one-shot"), which has one stable state and one unstable state, returning to the stable state after a set time following a trigger. Option C describes a logic gating function, not the fundamental operation of a multivibrator.