Which of the following describes a variable ratio schedule?

A variable ratio schedule refers to a reinforcement strategy where rewards are given after an unpredictable number of responses or purchases. This type of schedule is characterized by its unpredictability, meaning that the consumer cannot anticipate when the next reward will be received. This creates high and consistent levels of engagement because consumers continue to make purchases in hopes of receiving a reward, similar to gambling scenarios where payouts happen at random intervals.

In this context, the other options describe different reward structures. For example, a set number of purchases for a reward suggests a fixed ratio schedule, while consistent buying patterns imply a more predictable, perhaps fixed, approach to rewarding; these approaches do not capture the essence of variability and uncertainty inherent in a variable ratio schedule. The timed schedule option does not relate to behaviors based on the number of responses but rather on time intervals, which is a separate reinforcement concept.