Final answer:
In classical conditioning, the conditioned response (CR) is strengthened during the acquisition phase by repeated pairings of the conditioned stimulus (CS) and unconditioned stimulus (US), until the CR is reliably elicited by the CS alone.
Step-by-step explanation:
In classical conditioning, the conditioned response (CR) becomes established and is strengthened when the conditioned stimulus (CS) and the unconditioned stimulus (US) are paired many times during acquisition trials. This process is crucial for the CR to be reliably elicited by the CS alone. During this phase of learning, known as acquisition, every pairing of CS (e.g., a bell) and US (e.g., food) makes the CR (e.g., salivation in response to the bell) gradually stronger. Ivan Pavlov's classical experiments with dogs and the salivation response to the conditioned sound of a bell exemplify this process.
With repeated pairings, the organism, such as a dog in Pavlov's experiment, begins to anticipate the US after exposure to the CS, and the CR starts to occur in response to the CS alone. The initial period of learning, acquisition, sees the CR getting stronger through repeated CS-US pairings. The correct answer to the original question is (c), that the CS and US are paired many times during acquisition trials.