Illusion. (n.) a false idea; something that one seems to see or be aware of that really does not exist.
The best example of a primary source among these options that you've mentioned is an interview with her dad who is a Vietnam veteran. She can get all the details she needs from a person who was actually there.
"Brennan on the Moor" is structured in this way, so that the longer stanzas can follow the tale of Brennan and give more details to the story. The shorter stanzas act as a chorus, which is usually used to come back to the main idea of a song or poem (in this case, the idea that Brennan is the hero of the poem).
I would go with the author points out that the eyes reveal the essential characteristics of a person.