Classical conditioning is best defined as:

Classical conditioning is indeed best defined as learning by the process of association. This fundamental concept in behavioral psychology involves creating an association between a neutral stimulus and an unconditioned stimulus that naturally elicits a response. Over time, the neutral stimulus becomes a conditioned stimulus capable of triggering that same response on its own.

For instance, if a bird hears a particular sound every time it is about to be fed, it may eventually begin to associate that sound with receiving food, leading it to respond by showing signs of excitement or eagerness even when the sound is presented without food. This association between the sound (neutral stimulus) and feeding (unconditioned stimulus) exemplifies the principles of classical conditioning, highlighting how organisms can learn to adapt their responses based on experiences and environmental cues.

In contrast, the other choices focus on different learning processes. Memorization pertains to retaining information through repetition, which is not the core characteristic of classical conditioning. Learning by observation, often associated with social learning theory, involves acquiring behaviors by watching others, and does not involve the direct association aspect of classical conditioning. Lastly, learning through punishment is more aligned with operant conditioning, where behaviors are modified based on the consequences they produce, which again deviates from the core