The extent to which the conditioned stimulus (cs) and unconditioned stimulus (us) occur close together in time reflects

contiguity

The sequential occurrence or proximity of stimulus and response, causing their association in the mind. In the theory of contiguity, psychological theory of learning emphasizes that the only condition necessary for the association of stimuli and responses is that there be a close temporal relationship between them.

I believe the answer is: Contiguity
The theory of contiguity see that the association between conditioned stimulus and unconditioned stimulus could only exist if there is a close temporal relationship with the response So, for conditioned stimulus to exist, the unconditioned stimulus need to be followed by a specific response within a short time frame, otherwise an association would not be created.