Which buoy may be even numbered?

The unlighted nun buoy is the correct answer because it is designed to be even-numbered in the IALA (International Association of Lighthouse Authorities) buoyage system used in regions A and B. In this system, nun buoys, which are conical in shape, are typically even-numbered and colored red in region A and green in region B.

This pattern follows the convention that even-numbered buoys are associated with the starboard side of a navigable channel when approaching from seaward, aligning with the standard markings to assist mariners in navigating safely. Mid-channel buoys do not follow a numerical system based on even or odd numbers but are rather categorized based on their specific function, which can include being unmarked or identified simply by their shape and color. Lighted green buoys are typically used in tandem with the numbered identification system, but again, they are specifically identified as odd-numbered.

Therefore, the classification of the unlighted nun buoy as even-numbered provides a clear distinction for mariners navigating through channels, ensuring a systematic and standardized approach to buoy identification.