<b>Short answer: </b> It's not really an appropriate question for an interview situation.

I've been a programmer for 20 years and I couldn't answer this off the cuff in an interview.  I would find it one of those "WTF?" type questions that doesn't make any sense to ask.  It seems to me to be more of one of those over-a-coffee-in-the-lunchroom type conversations you may have with a co-worker.  

I'm struggling to see what value there is.  What are you trying to determine with it?  How will identifying a specific piece of code inherently improve your perception of the candidate?  If you want this, ask the poor candidate to bring something with them to the interview, or beforehand.