Final answer:
The correct action when entering or exiting a roundabout is to yield to all vehicles already in the roundabout, ensuring smooth and safe traffic flow.
Step-by-step explanation:
When entering or exiting a roundabout, you must yield to all vehicles already in the roundabout. This rule ensures that traffic flows smoothly and safely, as vehicles inside the roundabout have the right of way. It's important to approach the roundabout at a safe speed and prepare to yield, rather than coming to a complete stop or speeding up to merge. Using the horn is generally not necessary unless you need to alert others to prevent an accident.