Which action is indicative of conditioned response after successful conditioning?

The correct answer highlights the essence of classical conditioning, where a conditioned response is learned through the association of a neutral stimulus with an unconditioned stimulus. After successful conditioning, an organism begins to exhibit a response that is specifically linked to the conditioned stimulus rather than other stimuli.

In this context, "Response only to the conditioned stimulus" indicates that the organism has learned to associate the conditioned stimulus with a particular outcome, demonstrating that it has internalized this association. This specificity of response is a hallmark of successful conditioning, illustrating that the learned behavior is distinct and directed.

Other answers can be discussed for context; for example, increased avoidance of the conditioned stimulus might indicate an unconditioned response rather than a learned one, and immediate reactions to any stimuli lack the specificity required in conditioned responses. Returning to unlearned behavior reflects a loss of learned behavior rather than demonstrating a conditioned response. Therefore, the correct answer correctly encapsulates the principle of how conditioned responses are developed and displayed after learning has occurred.