The correct answer is option (D):
Alliance
The correct answer is alliance. An alliance is a formal agreement or treaty between two or more parties, often nations, for cooperation and mutual benefit. This directly aligns with the definition provided in the question: "A union or association formed for mutual benefit." The other options do not fit this definition. Communion refers to a religious practice, ratification is the act of approving something, a trustee manages assets for someone else, and a society is a broader term for a group of people.