Final answer:
When the Commanding Officer is ashore, the Union Jack is flown. This flag represents the CO's presence and is distinct from other British ensign flags that have different maritime uses.
Step-by-step explanation:
When the Commanding Officer (CO) is ashore, the Union Jack is flown to represent their presence. The Union Jack is the national flag of the United Kingdom and is commonly associated with the authority of the CO when they are off the ship and on land. Each of the ensigns mentioned—White, Blue, and Red—has specific uses and is tied to different types of British ships or maritime organizations. The White Ensign, for example, is used by the Royal Navy, the Blue Ensign by the Royal Fleet Auxiliary and government departments, and the Red Ensign by British merchant and passenger ships.