Answer:
C. By using a variety of biblical allusions
Explanation:
Given that appealing to ethos means that a speaker or author is illustrating or portraying himself as a credible or reliable source that people should believe or listen to.
Hence, when FDR was using a variety of biblical allusions in his first inaugural address, he was trying to depict that he knows what he's saying and people should listen to him, thereby appealing to ethos.
Therefore, in this case, the correct answer to the question is option C "By using a variety of biblical allusions."
