Which vessel characteristic indicates that it is over 50 meters in length?

The vessel characteristic indicating that it is over 50 meters in length is the presence of two masthead lights. According to the International Regulations for Preventing Collisions at Sea (COLREGs), specifically Rule 23 concerning the lights and shapes of vessels, a vessel that is over 50 meters in length must exhibit two masthead lights, placed vertically.

This configuration helps in identifying the size and type of the vessel from a distance, which is crucial for navigational safety. The additional masthead light serves as an important visual signal for other vessels, allowing them to determine the nature of the vessel they are encountering. Understanding these requirements ensures that mariners can assess potential navigation risks based on the observed lighting patterns of nearby vessels.

The other options do not provide reliable indicators of a vessel's length. For instance, one masthead light is appropriate for smaller vessels, and the presence or absence of stern lights and sidelights does not specifically correlate to vessels being over or under 50 meters in length.