Which best explains the purpose of the rhetorical device used in the following sentences?
we must learn from our mistakes. we must keep our sights on our goal. we must persevere.
a. to build the author's ethos by referring to credible sources
b. to create a memorable image by comparing two unlike objects
c. to prove a point by highlighting the obvious
d. to make the main points more memorable through repetition

The correct answer should be D. To make the main points more memorable through repetition. 

This isn't appealing to ethos because ethos is appealing to credibility. It is questionable whether or not the image itself is memorable or not. 

Repetition and parallelisms in speeches are used to make something more memorable and quotable. It is supposed to persuade you and this kind of sentence manipulation enables good diction.