In both regions of the IALA buoyage system, which topmark shown in illustration D022NG below is used on a special mark?

Correct Answer: Option D — D

Special marks are used to indicate areas such as spoil grounds, military exercise zones, or cables. In both IALA-A and IALA-B systems, these marks are yellow and feature a single yellow 'X' (St. Andrew's Cross) as a topmark. Illustration D022NG identifies this specific shape as the standard topmark for special marks.