What signal does a vessel not under command display during the day?

A vessel not under command, which is unable to maneuver as required by the ColRegs due to exceptional circumstances, displays two black balls during the day. This visual signal indicates to other mariners that the vessel is restricted in its ability to navigate, and they should take appropriate caution and avoid it if possible.

The use of two black balls is universally recognized in maritime navigation and serves a crucial communication function, alerting nearby vessels of the situation. This signal is a clear and concise way to indicate the vessel's status without the need for verbal communication, which might not be possible or effective in open water scenarios.

The other choices are associated with different conditions or signals used in maritime navigation. One black ball is used by vessels aground, three red lights signify a vessel at anchor, and the combination of a black and white flag does not correspond to any standard signal for vessels not under command. Understanding these signals is essential for safe and effective navigation on the water.