When ships are underway, which of the following customs should be observed?

The custom of flying the national ensign from the jackstaff when ships are underway is an important tradition that signifies the ship's national affiliation and sovereignty. The national ensign represents the nation to which the ship belongs and is a vital symbol of pride and identity for military and naval vessels. Flying the flag at the jackstaff, which is located at the front of the ship, is a standard practice that indicates the ship is in an operational status and acknowledges its presence on the water.

This practice not only promotes recognition of the ship's nationality but also serves to maintain proper decorum and etiquette in maritime operations. When ships are underway, it's essential to adhere to such customs for both national pride and the sake of maintaining good relations with other vessels on the water, as flags can serve as a point of communication about a ship’s identity and mission.

The other options outlined do not align with established maritime customs and protocols for ships that are operational and underway. Not flying any flags, or only displaying state flags, would not reflect standard naval practices, which emphasize the importance of displaying the national ensign to represent the ship's home country while at sea.