Which shape can typically be used by vessels at anchor to signal their status?

The shape that is typically used by vessels at anchor to signal their status is a ball. This signaling method is established under the Navigation Rules, which dictate the use of specific shapes to communicate the operational status of a vessel to other mariners.

When a vessel is anchored, it is required to display a single all-round white light or, if not underway at night, the ball shape provides a visual cue during the day. The presence of this shape helps other vessels understand that the anchored vessel is not in motion, thus allowing for safer navigation and communication on the water.

Other shapes mentioned, like rectangles, triangles, or cylinders, do not have defined roles in signaling a vessel's status at anchor according to the regulations. The use of the ball is specific to indicating that a vessel is anchored, making it the correct answer to this question.