What indicates a buoy that should be left to port when entering from seaward?

A buoy that should be left to port when entering from seaward is indicated by an odd number. This is a fundamental aspect of the IALA (International Association of Lighthouse Authorities) buoyage system, specifically the IALA Region B system (used in the United States).

According to these buoyage rules, navigational aids are marked in a way that allows mariners to easily recognize which side of the channel to navigate on. The odd-numbered buoys are designated as "nun buoys," which have a specific shape (typically conical) and are colored red. These markers indicate to vessel operators that they should keep these buoys on their port (left) side when entering a channel from the sea.

The other options present different characteristics of buoys but do not specifically indicate which side of the channel should be kept clear. While a white light or a group flashing characteristic may describe the lighting pattern of a buoy, and a nun shape refers to the buoy's physical design, it is the odd number designation that clearly communicates the critical navigational instruction regarding the position relative to the vessel's course.