Explain ‘extension’ with regards to classical conditioning.

Classical conditioning is the learning process resultant from the combination of two stimuli, one neutral and one strong, with the purpose to create the same response that takes place with the strong stimulus when the neutral stimulus is given. I believe you wanted to write "extinction" instead of "extension". Extinction, in context of classical conditioning, happens when a conditionate stimulus is presented without the unconditioned stimulus for long enough that eventually the conditionate response is not given anymore when the conditionate stimulus is presented.

Answerendoderm;