First, Pedro is a guy (guy = chico). So of course, his nationality would end by "o" not "a", so it's either Son Chilenos, either Yo soy Chileno.
Since Pedro is only one person, So his nationality shouldn't end by "s".
We still got one choice: Yo Soy Chileno. "Yo" can be removed too, and simply say "Soy Chileno".
Hope this Helps! :)