Under what condition can a foreign national ensign be displayed at the mainmast by the U.S. Navy?

The display of a foreign national ensign at the mainmast by the U.S. Navy is appropriate during specific ceremonial practices, and the correct answer highlights one of those significant occasions. When the Navy fires a salute to a foreign nation upon entering port, it demonstrates respect and acknowledgment of the host nation’s sovereignty and customs. This gesture is not only a formality but is also rooted in international naval traditions, whereby vessels honor each other's flags through the exchange of salutes.

Displaying the foreign ensign during the salute signifies the recognition of the foreign nation and its maritime customs, which is an important aspect of diplomatic relations at sea. This action reinforces the camaraderie between the Navy and the foreign nation and helps to promote goodwill.

In other situations, such as during flag-raising ceremonies or joint military exercises, while the foreign ensign may indeed be involved, it wouldn't typically be displayed at the mainmast. In foreign waters, the protocol could be different and often depends on specific naval regulations or multilateral agreements. Thus, the condition under which a foreign national ensign can be displayed at the mainmast is most appropriately encapsulated by the act of firing a salute upon entering port.