Answer and explanation:
First, we have the word Athens. Then, we get to know that Athenian is a person from Athens. The final -s has been removed in Athens, and -ian was added. We can assume that -ian is the suffix, therefore. This suffix most likely indicates origin, a person's place of birth. Other words that use the same suffix and that indicate the origin of someone are: Parisian (Paris), Brazilian (Brazil), Italian (Italy), Argentinian (Argentine), among others.
