Final answer:
The proper action when approaching a roundabout is to slow down to safely enter the traffic flow and yield to cars already in the roundabout.
Step-by-step explanation:
When approaching a roundabout, the correct action to take would be D) Slow down. This is because a roundabout is a traffic management system designed to promote a smooth flow of traffic by reducing speed and encouraging a circular rather than straight-through movement.
Speeding up would be dangerous and counterproductive, whereas stopping (unless the traffic within the roundabout necessitates it) or turning on your flashers is generally unnecessary. One should approach with caution, checking for oncoming traffic within the roundabout, and yield to any drivers already circulating.