Final answer:
The World Customs Organization (WCO) is responsible for establishing classification numbers for importers and exporters, not the World Trade Organization (WTO), the International Monetary Fund (IMF), or the United Nations (UN).
Step-by-step explanation:
The organization responsible for establishing classification numbers that must be used by importers and exporters is the World Customs Organization (WCO). Although the World Trade Organization (WTO) provides a venue for trade negotiations and establishes rules governing international trade, it does not assign these classification numbers. Whereas the International Monetary Fund (IMF) deals with international financial transactions and the United Nations (UN) encompasses a wide array of global issues, neither are involved in the classification of imports and exports. The correct answer to the question is Option 4, the World Customs Organization (WCO).