Answer:
It emphasizes the narrator's feeling of anxiety.
Step-by-step explanation:
Describing the diving board as "lonesome" shows that the narrator is nervous about going onto it all by her/him self and having to jump off it.
ALSO...
I just took the test. I chose "It describes the appearance of the diving board", but it was wrong and the correct answer is "It emphasizes the narrator's feeling of anxiety.
HOPE THIS HELPS!!! :)