Which of the following is true about the national ensign when a vessel is not underway?

The national ensign, which is the flag representing the United States, can remain displayed when a vessel is not underway. This practice shows respect for the flag and signifies the vessel's presence, even when it is stationary. Displaying the ensign continuously during non-underway periods reflects adherence to naval customs and courtesies, allowing the flag to be flown proudly.

In contrast, the other options do not accurately reflect standard practices regarding the national ensign. For instance, taking the ensign down at night (the first option) is not a requirement; many vessels choose to keep it flying. The notion that the flag must be half-masted at night (the third option) does not apply universally and is only pertinent when there is a specific directive to do so in remembrance or honor. Lastly, stating the ensign is only displayed in port (the fourth option) is incorrect, as the flag can be flown at sea as well when the vessel is anchored or moored.