Final answer:
The international organization with membership by invitation only is the World Travel and Tourism Council (WTTC).
Step-by-step explanation:
The organization that is an international organization with membership by invitation only is the World Travel and Tourism Council (WTTC). The WTTC is comprised of business leaders in the travel and tourism industry; membership is by invitation to CEOs and other high-ranking executives. In contrast, the World Tourism Organization is a specialized agency of the United Nations, Visit Florida is a state-specific tourism organization, and the International Air Transport Association is a trade association of the world's airlines. For additional information about organizations with a focus on sustainable tourism development, you can look into The International Ecotourism Society.