Please answer the following question by reasoning step-by-step.
One might find theirs open, with the letters that had been delivered stolen.\nOptions:\n- front door\n- newspaper\n- post office\n- package\n- mail box

mail box
Mailbox is a box into which mail is delivered, especially one mounted on a post at the entrance to a person’s property.

One might find theirs mailbox open, with the letters that had been delivered stolen.

.

So, the answer is mail box