Final answer:
For strong, long-standing trade relationships, sales on open account are the preferred method of financing because it is based on trust and allows payment after delivery, rather than requiring immediate payment or extensive documentation.
Step-by-step explanation:
If the exporting and importing parties have a strong, long-standing relationship, the method of financing they would most likely use is D) sales on open account. This method allows the importer to receive the goods and services with an agreement to pay the supplier at a later date, without the need for immediate payment at the time of shipment or advanced documentation. This is based on mutual trust developed over time through a history of reliable transactions between the two parties.
In comparison, other methods like documentary credit (letter of credit), documentary collection, cash in advance, and bank draft provide varying degrees of payment security and are commonly used when the level of trust between the trading partners is lower or when dealing with new or less familiar counterparties.