As the car travels around the roundabout, the characteristic that is constant is speed. While the direction and velocity (which includes both speed and direction) are changing as the car goes around the curve, the speed remains constant if we assume there are no changes in acceleration or deceleration. However, in a strict physics sense, if you take the definition of velocity into account (which combines both speed and direction), then technically the velocity is not constant either, due to the change in direction.
Thus, the most appropriate choice, assuming no change in the magnitude of speed, is speed.