Which of the following is a requirement for the placement of boat registration numbers?

The requirement for boat registration numbers to be at least 3 inches high ensures that they are easily visible and legible from a distance. This visibility is crucial for law enforcement and other boaters to identify the vessel appropriately. The height requirement helps to standardize how registration numbers are displayed, promoting safety and compliance with regulations.

While it's essential for registration numbers to be displayed on both sides of the boat for visibility from various angles and distances, the critical aspect of the height ensures they can be seen clearly. Other considerations such as illumination and cursive writing do not align with standard practices for registration number placement, which prioritize clarity and legibility above all else.