What defines a transaction between trading partners?

A transaction between trading partners is characterized by a transfer of product that involves a change of ownership. When a trading partner acquires products from another entity, it is essential that ownership of those products is transferred in order for the transaction to be defined as such. This is a fundamental concept in commerce and trade, as ownership transfer establishes accountability, responsibility, and legal rights over the products during and after the transaction.

In contrast, some of the other options might describe interactions related to product handling but do not fulfill the criteria of ownership change. For instance, a transfer of product without change of ownership does not constitute a true transaction between trading partners; rather, it can represent a consignment or loan scenario. Similarly, a purchase agreement between pharmacies may describe an intention to transact but does not itself define the act of transferring ownership. Finally, a distribution of products among hospitals suggests a logistical aspect of health care operation but again does not focus on the critical element of ownership transfer that defines a transaction. Hence, the emphasis on change of ownership in option C makes it the correct answer.