The characteristic that is constant for the car as it travels around the roundabout is speed.
As the car moves with a constant speed of 10 meters per second, its speed remains the same. However, the direction of the car is constantly changing as it moves around the roundabout, which means the velocity (a vector quantity that includes both speed and direction) is not constant. The position of the car is also changing as it moves around the roundabout. Therefore, the correct answer is speed.