What is typically flown by civilian ships to indicate nationality?

Civilian ships typically fly a merchant ensign to indicate their nationality. The merchant ensign is a flag that represents the country under whose laws the ship is registered and serves as a symbol of its commercial nature. This flag is distinct from a naval ensign, which is used by military vessels to indicate their affiliation with a nation's navy.

The merchant ensign is important for several reasons, including legal implications at sea, recognition by other vessels, and for determining the rights and responsibilities associated with the ship's nationality under international maritime law. By displaying the appropriate merchant ensign, a civilian ship can be easily identified as part of the merchant fleet of a specific country, which supports navigation and international relations at sea.

The other options do not serve the same purpose as the merchant ensign. The naval ensign is reserved for military vessels and denotes a ship's military status. A customs flag is generally used for customs purposes and not specifically to indicate nationality, while signal flags are used for communication between ships rather than denoting nationality.