A. "When she ... lived there."
This line only defines a specific detail about how the special agent survived through a fake name as a spy. No central theme is described through this sentence.
B. "Baissac’s goal ... resistance groups."
This statement provides the agent’s motive and the way she enacted the task provided. However, that does not completely cover everything in the passage.
C. "Baissac did ... her tasks."
Significant work is not specific, and Normandy and traveling by a bicycle are smaller and irrelevant details, not the big picture that should be concluded from this passage.
D. "As a ... German troops."
This is the statement that definitely defines the central idea of the excerpt. When we break this line into sections, we can see that it illustrates that she performed multiple essential tasks when appointed in Normandy. The phrase “sometimes dangerous tasks” describes the critical nature of the job she handled in there. And, also the opposition (German troops) is clearly mentioned in this sentence which helps to convey the idea very clearly.
D. a series of events that build the plot toward the climax
