Final answer:
Erin has used the foot-in-the-door effect to have her brother agree to a series of increasing favors, starting with orange juice and escalating to making a full breakfast.
Step-by-step explanation:
The psychological concepts Erin has used on her brother are D). Low-ball technique and foot-in-the-door effect.Erin started by asking her brother for a small favor, getting a glass of orange juice, which he completed. This is a classic example of the foot-in-the-door technique, where a person agrees to a small request which increases the likelihood they will agree to a subsequent larger request.As he was about to fulfill the second request, which was slightly larger, she changed the request to an even larger task, asking for bacon and eggs instead of cereal. This maneuver, where terms are changed after an initial agreement is reached, is related to the low-ball technique, though it isn't a perfect example, since that usually involves making the initial request seem more attractive than it actually is.In summary, Erin cleverly convinced her brother to do her a series of increasing favors by initially asking for something simple, and then gradually increasing her requests.