Answer :
You'll use ser when you're talking about permanent states or characteristics. Like when someone is smart. They aren't smart sometimes, but all the times.
ex) I am - yo soy
Answer:
de
Explanation:
De more or less means of, so when you say ser de ___, you are saying they are of that place. Just saying ser would be incorrect because you would be saying they are that place, not that they are from that place.