Which action must a driver take when they encounter a roundabout?

When a driver encounters a roundabout, yielding to traffic that is already in the roundabout is a critical action to ensure the safe and efficient flow of vehicles. This action is based on the priority rules of roundabouts, where vehicles inside the roundabout have the right of way. By yielding, the driver allows for a smoother merging of traffic and helps prevent collisions that could occur from attempting to enter the roundabout without yielding.

Additionally, entering without stopping, eliminating the use of turn signals, or speeding up to pass through the roundabout can lead to confusion and potential accidents. It's essential for drivers to signal their intentions when approaching and exiting a roundabout to communicate with other road users, thus enhancing safety and reducing the risk of misunderstandings.