Answer:
Martin Luther King was an important civil rights activist. He was a leader in the movement to end racial segregation in the United States. In 1968, shortly after Martin Luther King died, a campaign was started for his birthday to become a holiday to honor him.
Explanation:
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.