As the car travels around the roundabout, the characteristic that is constant is speed.
While the car's direction is constantly changing as it moves around the curve, if it is maintaining a steady, unchanging speed (i.e., no acceleration or deceleration), then speed remains constant. However, velocity, which is a vector quantity that includes direction, is not constant due to the continuous change in direction. Position also changes as the car moves around the roundabout.
So, the correct answer is Speed.