The association between a UCS and a CS can be extended to include more stimuli (more stimuli related to the CS can elicit the CR). What is this phenomenon called?

The answer is Stimulus generalization

Explanation:

Stimulus generalization is an example of classical condition. Classical conditioning takes a stimulus that does not cause a particular response (neutral stimulus) and then pairs it repeatedly with an unconditioned stimulus that will cause an unconditioned response. In the case of Stimulus generalization, I will give an example of a subject presenting food to a dog once they ring a bell. Lets say that you have taught a dog to salivate every time it hears a bell ring. If you took another bell that has a similar sound and rang it, the dog would still salivate and come pick its food. This is a perfect example of Stimulus generalization. The dog has responded to a new stimulus as if it was the initial conditioned stimulus.