The law. You have access to a court and a judge. It exists and it should remain existent.
Wether it is fair or corrupt is another debate.
Discussion
That’s what, not who
Still, it is the only right answer.
The law secures your property. Property laws must be enforced by someone.
That's like saying the police is a person, it is not. Your question is semantically trivial towards wether the answer can be an institution, because the answer can't be a person.