Which of the following circuits continuously alternates between two states without an external clock signal?

An astable multivibrator is an electronic circuit that has no stable resting state. The prefix 'a-' means 'not', so 'astable' translates to 'not stable'. Because it cannot rest in either a high or low state, it continuously transitions back and forth between the two. This continuous alternation creates a steady stream of square waves, effectively functioning as an oscillator without needing any external clock signal to drive it. The frequency of this oscillation is typically determined by the charging and discharging of capacitors within the circuit. In contrast, a monostable multivibrator has one stable state and needs a trigger, while flip-flops (like J-K or T types) are bistable and generally require an external clock pulse to change states.