Translation:You are a common citizen like us.
My answer "an average citizen" was accepted, however indicated with a typo. "a average ..." was suggested instead...Could someone provide a native English explanetion for why the article is "a" and not "an" in this case?
It isn't. Your answer "...an average citizen" is the correct one. Duolingo's "...a average citizen" is completely wrong. It's probably just a typo. Please report it if you get the chance. :)
Commoner is more closely translated to "popolano", or in this case "popolana": member of the common people, of low standing in society. Common citizen here would be intended as a citizen like any other, with no special privileges; although "cittadina" is also a small town, but of course that meaning would be improbable here.
In the U.S. we would more likely say "an ordinary citizen," if this is what is meant.
I translated the sentence as, "you are an ordinary small town like us." Coming from a small, quaint town, it made more sense in English to me than "you are a common/ordinary citizen like us." oh well.....
...but in this sentence the subject is "tu" which is the familiar singular for "you," so probably addressed to one person.