Which one of the following is referred to as a free one-shot circuit?

The correct option is 2

Concept:multivibrator

• The reason is that a monostable multivibrator has one of its states stable, but the other state is temporary (transient), which can be induced by an external trigger.
• When this temporary state finishes (when a timing period has expired), the circuit will return to its stable state.
• This property of having a temporary active state initiated by a trigger pulse and returning automatically to the stable state, precisely makes it a one-shot or single pulse generator.
• In other words, it is capable of producing a single output pulse when it receives an input trigger, hence the name "one-shot" circuit.

Additional Information 

• A Monostable multivibrator has a stable state and a quasi-stable state. This has a trigger input to one transistor. So, one transistor changes its state automatically, while the other one needs a trigger input to change its state.
• A Bistable multivibrator has both the two states stable. It requires two trigger pulses to be applied to change the states. Until the trigger input is given, this multivibrator cannot change its state.
• An Astable multivibrator is such a circuit that it automatically switches between the two states continuously without the application of any external pulse for its operation. As this produces a continuous square wave output, it is called as a free-running multivibrator.