Final answer:
The initial phase of conditioning when the CS and US are paired is known as acquisition, a critical period for learning the conditioned response in classical conditioning. Hence, option (a) is correct.
Step-by-step explanation:
The term for the initial phase of conditioning when the conditioned stimulus (CS) and the unconditioned stimulus (US) are paired in classical conditioning is known as acquisition. During the acquisition phase, a neutral stimulus begins to be associated with an unconditioned stimulus so that it will eventually elicit the conditioned response even without the presence of the unconditioned stimulus. This period is crucial for learning the conditioned response.
For example, in Pavlov's classic experiment with dogs, the ringing of a bell (CS) was initially a neutral stimulus. When paired with the presentation of food (US), the dogs began to associate the bell with the arrival of food. Over time, the bell alone triggered salivation (CR). Acquisition represents the establishment of this learned association, crucial for understanding the formation of conditioned responses in classical conditioning paradigms.