Answer:
D. The unrhymed lines in each stanza emphasize the main idea of the poem: missing a friend who has moved away.
Explanation:
This is a one-sided sad story (so not a or b) and they are only missing the other friend, not saying anything about them. (so not c)
Therefore the answer would be D! It doesn't rhyme and they are missing a friend which makes them v sad.
Hope this helps!
(I also hope i'm not late haha)