Explanation: An astable multivibrator is a free-running oscillator that continuously switches its transistors between cutoff and saturation. In a common-emitter configuration, when a transistor is in cutoff, its collector voltage is high (near the supply voltage). When the transistor is saturated, its collector voltage is low (near ground). The regenerative feedback in an astable multivibrator causes each transistor to rapidly switch between these two states. This abrupt, periodic transition between two distinct voltage levels at the collectors, which are held for a duration before switching again, produces a square wave. A sine wave requires a linear, resonant circuit. A sawtooth wave involves a linear voltage ramp, not the sharp, instantaneous transitions of a switching circuit. Half-wave pulses are typically short, unipolar events, unlike the alternating high and low sustained states of a square wave.