Concept:A bill of exchange is a written order used to make payments in international trade.Explanation:A bill of exchange is an instrument that directs one party to pay a fixed sum of money to another party on demand or on a specified date.It is widely accepted in international trade because it provides a secure and documented method of payment between buyers and sellers in different countries.Options like a promissory note are also payment instruments, but a bill of exchange is the most appropriate and commonly used instrument for international transactions.I.O.U. and postal order are not standard instruments for international trade due to limited acceptability and security.Answer:D. Bill of exchange