Final answer:
An illusory correlation is the false perception of a relationship between two variables when none exists, often influenced by the availability heuristic and confirmation bias.
Step-by-step explanation:
An illusory correlation occurs when individuals perceive a relationship between two variables when, in fact, no such relationship exists. A classic example of this would be the belief that the phases of the moon have a direct impact on human behavior, despite the lack of scientific evidence to support this claim.
Therefore, according to the textbook, the correct definition of an illusory correlation is: b) an illusory correlation occurs when there is no evidence for a relationship between two variables, which then promotes greater use of the availability heuristic.