What type of vehicles must yield to others at a roundabout?

The correct choice is that all entering vehicles must yield to existing traffic at a roundabout. This rule is designed to ensure smooth and safe traffic flow within the roundabout. When vehicles are approaching a roundabout, they must wait for vehicles already circulating to pass by before entering. This practice helps to minimize conflicts and accidents, as it allows those already in the roundabout to maintain their speed without interruption.

In contrast, while large trucks and emergency vehicles have their own sets of rules regarding maneuverability and may have different clearance requirements, the fundamental principle for roundabouts applies universally to all entering vehicles regardless of their size. Understanding this principle helps drivers navigate roundabouts safely and efficiently.