Sailing vessels less than 12 meters (international) or 20 meters (inland) in length must display red and green sidelights, or a red and green _________________________ light, along with a stern light.

Sailing vessels less than 12 meters in length (international) or 20 meters (inland) are required to display red and green sidelights, which indicate their orientation and direction to other vessels. When a vessel opts for a combined light instead of individual sidelights, this light serves the same purpose as having separate red and green lights but is designed as a single unit.

The combined light displays both red and green colors, allowing it to fulfill the regulatory requirement for sidelights while simplifying the lighting setup on the vessel. This can be particularly advantageous for smaller vessels, where space and simplicity are a consideration.

Each of the other options represents a type of light that does not meet the necessary requirements for sidelights. Flashing lights, strobe lights, and rotating lights serve different functions, primarily in signaling or emergency situations, and are not appropriate for indicating a vessel's direction in normal operating conditions. Thus, the combined light is the proper solution, providing an efficient way to signal the vessel’s heading while complying with navigational safety regulations.