In classical conditioning, during extinction the is consistently omitted and the gradually disappears.

The correct answer is that; during extinction, the unconditioned stimulus is consistently omitted and the conditioned response gradually disappears. 

In classical conditioning, an unconditioned stimulus is paired with a conditioned stimulus to elicit a conditioned response. Meaning, the unconditioned stimulus normally elicits the response but once its paired with a conditioned stimulus, both of which will elicit the same response. The response is now called the conditioned response. Once the unconditioned stimulus is consistently omitted, the conditioned response will the disappear gradually.

The answer - c apex