Suppose we want to find an event that will serve as an effective reinforcer for a given person. According to the disequilibrium principle, we should begin by determining

A) which brain areas are more active than average for this person.
B) which other people this person admires and wants to resemble.
C) how old this person is, and how old are his or her friends.
D) which behaviors the person has recently not had an opportunity to do.

The disequilibrium principle states that an effective reinforcer creates a state of imbalance or disequilibrium in the individual, motivating them to engage in behaviors that will restore equilibrium. Therefore, determining which behaviors the person has recently not had an opportunity to do can help identify potential reinforcers that will create a state of disequilibrium and be effective in motivating behavior. Options A, B, and C are not relevant to identifying effective reinforcers based on the disequilibrium principle.