Third person plural pronoun is THEY.
If you want to create a possessive pronoun out of that one, that would be THEIRS.
Possessive pronouns, as the name itself says, determine some kind of possession. For example:
Is this books yours? - No, it's theirs.
I would say a) joy but i’m not a 100% sure
I would say the correct answer is the native Americans' peaceful 9 months occupation of Alcatraz island.
