Millie doesnât like waiting in line. Whenever she goes to her local supermarket, she has to wait at least 20 minutes in line. So, she decides to buy all her groceries online instead. In this scenario, Millie is demonstrating:

operant conditioning

Explanation:

Operant conditioning: In psychology, the term "operant conditioning" was proposed by one of the famous behaviorists and psychologists named B. F. Skinner and is also denoted as instrumental conditioning.

According to Skinner, operant conditioning is determined as one of the different learning methods that generally arises through punishments and rewards process of a particular behavior. An organism, therefore, makes an association or connection between a specific behavior and its consequence via operant conditioning.

In the mentioned scenario given in the question above, Millie is demonstrating "operant conditioning".