Answer:
A. Sentence 3
Explanation:
Sentence 3, "In Sophocles's Greek tragedy Antigone, the characters must decide whether to stick to their beliefs or allow themselves to change," most clearly gives background information. This sentence introduces the reader to the discussion's context by mentioning the Greek tragedy "Antigone" by Sophocles and providing a brief overview of the central theme—the characters' dilemma of sticking to their beliefs or embracing change. It establishes the play and the thematic framework, serving as background information for the reader before delving into the essay's main points.