Which of the following circuits continuously alternates between two states without an external clock signal?
Why A multivibrator's name tells you how many stable states it has. An astable multivibrator has no stable state, so it flips back and forth on its own at a rate set by its RC time constants, making it a free-running square wave oscillator that needs no external clock. Flip-flops, by contrast, are clocked (synchronous) devices that only change state when a clock edge arrives.
Watch out The monostable multivibrator is the tempting one, but it has one stable state: an external trigger kicks it into the other state for a fixed time and it returns, which is why it is called a one-shot.
A-stable = not stable = never settles, so it oscillates. Mono-stable = one stable state = one-shot.