Final answer:
The speaker uses the rhetorical device to inspire hope and underscore that difficult times can reveal opportunities for positive change, similar to how stars are visible only in the dark.
Step-by-step explanation:
The rhetorical device in the sentence, "But I know, somehow, that only when it is dark enough can you see the stars," serves to connect to the audience on an emotional level and to point out the possibilities that arise from difficult times. The speaker uses it to inspire hope and persistence during challenging periods, suggesting that challenges can reveal the opportunity for positive change, much like stars that become visible only in the darkness.
Therefore, the correct answer is: c. He uses it to help convince the audience to fight for civil rights by pointing out in an original and powerful way the real possibilities beyond the troubled times they live in.
This metaphor is powerful and underscores that it is precisely during the darkest times that the true potential for change can emerge, which is particularly pertinent in the context of civil rights struggles.