What is a common feature of the jack staff when a vessel is in port?

A common feature of the jack staff when a vessel is in port is that it carries the union jack. This is a traditional practice in many naval customs, where the union jack is hoisted on the jack staff when a ship is alongside and not underway. The display of the union jack signifies the ship's nationality and adherence to naval protocol.

In addition to representing the ship’s nation, displaying the union jack while in port reinforces the maritime tradition and customs associated with naval operations. This practice is deeply rooted in naval history and varies among different countries, but it is widely recognized within the context of naval etiquette.

The other options, while relevant to various aspects of a ship's operations or customs, do not pertain specifically to the jack staff in the context of being in port. For instance, while lowering the jack staff might happen during certain maneuvers or in specific situations, it is not a common practice when simply docked. Displaying the national ensign usually occurs at the stern rather than at the jack staff, and flying the customs flag pertains to customs and immigration regulations and is typically displayed from the mainmast rather than the jack staff.