What type of vehicle must display a sign when using a public highway and travels at 25 MPH or less?

The correct choice is slow moving vehicles because they are specifically defined in traffic laws as vehicles that are incapable of maintaining speeds greater than 25 miles per hour under normal road conditions. This designation includes vehicles such as farm equipment, construction vehicles, and other machinery that may commonly operate at lower speeds.

In order to alert other drivers on the road, slow moving vehicles are required to display a sign, often a fluorescent orange triangle, to indicate their reduced speed. This sign serves to ensure that other drivers can adjust their speed and navigate safely around the slow-moving vehicle, thus enhancing road safety.

Other vehicle types, such as emergency vehicles, standard cars, and sports cars, are designed to operate at or above typical road speeds and do not have the same requirements for signage when traveling at lower speeds. Therefore, they do not require any special notification to other drivers in the same manner as slow moving vehicles do.