The correct answer is B. had finished.
"After she had finished her homework, Anaisha finally relaxed and watched a movie with her friends."
This sentence describes two past actions where one happened before the other. "Had finished" (Past Perfect tense) is used for the action that was completed first (finishing homework), while "relaxed and watched" (Simple Past) is used for the action that happened after. This sequence of events in the past requires Past Perfect + Simple Past structure.