Which shape indicates that a boat should keep this marker on the right side when passing?

The red triangle is used to indicate that a boat should keep this marker on the right side when passing. This is part of the visual system of navigation aids in boating, specifically the lateral buoyage system. In this system, red markers typically indicate the right side of a channel when entering from open water, which is crucial for safe navigation. This convention helps boaters identify the safe passage and avoid hazards in the water.

Understanding the color system is essential for all boat operators, as it ensures consistent communication about navigational routes and potential dangers. The other shapes, such as green squares, yellow circles, and white squares, do not convey the same meaning regarding channel navigation, which makes the red triangle the correct choice for indicating the right side passage.