What restriction must be followed when a non-licensed person speaks to a foreign station using an amateur station under the control of a licensed operator?

The correct answer is that the foreign station must be in a country with which the U.S. has a third-party agreement. This requirement is in place to ensure that communications through amateur radio comply with international regulations and agreements that govern amateur radio operations. The third-party agreement effectively allows non-licensed individuals to communicate via amateur radio to foreign stations, but only if the destination country has an arrangement with the U.S. permitting such exchanges.

This restriction is important because it maintains the integrity of amateur radio operations, ensuring that all parties involved are authorized to communicate in this manner. If the foreign station were in a country without such an agreement, it would not be permissible for a non-licensed person to communicate with them through an amateur station.

While it is true that all parties must adhere to proper identification protocols and that the operator must manage the use of the station, these do not inherently restrict the communication in the same way that the need for a third-party agreement does.