The stimulus which does not require any conditioning for the desired behaviour is called

A stimulus needing no conditioning to trigger the response is the unconditioned stimulus.

A conditioned stimulus gains its power only after pairing with the unconditioned one.

An unconditioned response is the natural reaction, not a stimulus.

A conditioned response is the learned reaction, not a stimulus.

In Pavlov's work, food was the unconditioned stimulus and salivation the unconditioned response.

Classical conditioning was demonstrated by Ivan Pavlov.