When a dual purpose marking is used, the mariner following the Intracoastal Waterway should be guided by the __________.

Correct Answer: Option C — shape of the yellow mark

The Intracoastal Waterway (ICW) uses yellow symbols on aids to distinguish them from standard lateral marks. When a dual-purpose mark is encountered, the yellow shape—a square for port or triangle for starboard—provides the definitive guidance for the mariner, as established in the USCG Navigation Center's ATON guidelines.