What signal appears during the day for a vessel not under command?

A vessel not under command is required to display specific day signals to indicate its status to other vessels. The correct signal for a vessel not under command during the daytime is two black balls, which is recognized under the International Regulations for Preventing Collisions at Sea (COLREGS).

This signal effectively communicates to nearby vessels that the vessel displaying it is unable to maneuver as required due to exceptional circumstances (such as an engine failure or other issues). The use of two black balls is clear and easily identifiable, ensuring that other mariners understand the vessel's inability to proceed normally and can take appropriate actions to avoid collisions or unsafe situations.

Understanding these signal requirements is crucial for ensuring safety and navigating effectively in maritime environments. The other options represent different vessels and their respective states, but for a vessel not under command, two black balls are the universally accepted day signal.