Which shape is associated with red buoys?

The correct answer is cone-shaped (nun). In the context of navigational aids, red buoys are typically identified as nun buoys, which are cone-shaped. These buoys indicate the right side of a channel when entering from open water. The cone shape easily distinguishes them from other types of buoys that might be present in a waterway.

Contrast this with the other shapes listed. Cylindrical buoys are generally used for other navigational purposes and are not specifically associated with the red color. Rectangular buoys do not adhere to the standard buoy shapes recognized for navigation. Flat shapes are less common in buoy design and do not signify the traditional meanings associated with navigational aids. Therefore, the cone shape is uniquely recognized as a hallmark of red buoys in nautical navigation.