According to behaviourist perspective, learning is primarily based on:

Behaviourism (associated with Watson, Pavlov, and Skinner) explains learning as an observable, measurable change in behaviour that results when a stimulus becomes linked to a response — through classical conditioning (a neutral stimulus paired repeatedly with another stimulus until it comes to trigger a response on its own) or operant conditioning (a response is strengthened or weakened by the reinforcement or punishment that follows it). This view deliberately leaves out unobservable internal mental processes as the mechanism of learning.

The question asks what behaviourists treat as the primary basis of learning. Applying the definition above: a stimulus and a response become connected through repetition or consequence, so that the response reliably follows the stimulus. "Association between stimulus and response" is precisely this stimulus-response bond, so it matches how behaviourist theory defines learning.

• Grasping ideas or relationships that go beyond direct sensory experience, and reorganising information mentally to understand them, describes cognitive processing rather than a stimulus-response bond.

• A state of psychological comfort, security, and positive affect describes a person's inner emotional state, not a link between an external stimulus and a behavioural response.

• Exchanges such as conversation, collaboration, or observing others describe interpersonal exchange, which is a different mechanism from a stimulus becoming linked to a response through pairing or consequence.

Only the stimulus-response bond matches the behaviourist mechanism of learning, so "Association between stimulus and response" is the answer — consistent with standard educational-psychology references (e.g., IGNOU/eGyanKosh material on behaviourism and OpenStax Psychology's treatment of classical and operant conditioning).