Which requirement applies to all vessels when under way in fog?

When vessels are underway in fog, there is a critical requirement to sound appropriate sound signals as part of maintaining safety and communication with other vessels. The International Regulations for Preventing Collisions at Sea (COLREGs) stipulate the necessity for vessels to emit sound signals in reduced visibility conditions to indicate their presence to nearby vessels. This helps prevent collisions by alerting others to the vessel's position and actions.

Using appropriate sound signals is vital because sound travels farther than visual signals in fog conditions, where visibility is severely limited. The specific requirements for sound signals include the use of a horn or whistle to produce blasts at specified intervals. This information is essential for navigating safely through such conditions, ensuring that vessels can determine each other's locations and intentions.

The other options relate to safety in fog but do not encompass the broader requirements established in maritime rules regarding communication and awareness while navigating in reduced visibility. Therefore, the obligation to sound appropriate sound signals stands out as a key requirement for all vessels underway in fog to enhance overall maritime safety.