The quote that best represents the main theme of "The Book of Unknown Americans" is:
B I spent a lot of time trying to find it in me, but usually I couldn't. I felt more American than anything, but even that was up for debate.... (paragraph 3)
This quote encapsulates the struggle of identity and belonging that is central to the experiences of the characters in the book, particularly as they navigate their immigrant status and attempt to reconcile their cultural heritage with their lives in America.