When observing another vessel at night, what lights indicate a head-on situation?

In a head-on situation at night, a vessel will exhibit both its sidelights and masthead lights in alignment when observed from the approaching vessel's perspective. This configuration occurs because each vessel is required to show its sidelights—green on the starboard side and red on the port side—as well as its white masthead light, which should be positioned at a height above the sidelights.

When these lights appear in a straight line, it signifies that the vessels are approaching each other directly. The alignment of the sidelights and masthead light creates a clear visual cue that helps mariners determine that a risk of collision exists, prompting them to reduce speed, alter course, or take appropriate measures to ensure safe navigation.

The other options do not provide the correct information about lights in a head-on situation. Bow lights alone would not be visible from another vessel directly ahead, and stern lights are only visible when approaching from behind—neither of which indicate a head-on encounter. Deck lights serve various functions but are not significant in navigating or determining the relative positioning of vessels at night.