Concept:The correct word is one that describes a person who signs an official agreement or treaty.Explanation:A "signatory" is a party that has signed an agreement, especially a state that has signed a treaty.This fits the meaning of the sentence perfectly.The other options do not match: a "partner" shares in an activity, a "guarantor" gives a formal promise, and a "participant" takes part in something.Only "signatory" refers specifically to the act of signing.Answer:B. signatory