Under the IALA Buoyage Systems, the topmark of a red and white vertically-striped buoy shall be __________.

Correct Answer: Option B — a single red sphere

IALA Safe Water marks are identified by red and white vertical stripes. Per IALA guidelines, these marks are topped with a single red sphere. This distinct shape and color configuration allows mariners to identify the mark as indicating safe water, often used for mid-channel or fairway markers.