Sadie's behavior can best be described as B) Habituation.
Habituation is a form of learning in which an organism decreases or ceases its response to a stimulus after repeated exposure. In this case, Sadie initially reacted to the letter carrier by barking and scratching at the door, but over time she has learned that the letter carrier's presence does not pose a threat or require a response, resulting in her now ignoring the visit altogether.