Classical conditioning is associative learning described by Ivan Pavlov. A biologically potent unconditioned stimulus (UCS, e.g. food) reflexively evokes an unconditioned response (UCR, e.g. salivation). When a neutral stimulus (e.g. a bell) is paired with the UCS many times, the neutral stimulus is transformed into a conditioned stimulus (CS).
The defining, observable outcome of successful conditioning is that this conditioned stimulus, presented alone, now triggers a learned reaction - the conditioned response (CR). So the phrase that captures the learned phenomenon is "conditioned stimulus producing a conditioned response."
The other choices are distractors: a UCS evoking a UCR is just the pre-existing reflex; describing the link as CS-CR or UCS-UCR misrepresents the mechanism, since the underlying association formed during acquisition is between the CS and the UCS.
\[\boxed{\text{CS} \rightarrow \text{CR (conditioned stimulus produces conditioned response)}}\]