The characteristic that is constant for the car as it travels around the roundabout is b. speed.
While the car is traveling in a circular path, its speed remains constant if it is not accelerating (i.e., maintaining a constant distance from the center of the roundabout). However, since the car is changing direction, its velocity (which includes both speed and direction) is not constant. Position is also changing as the car moves around the roundabout, and direction is continuously changing.