Final answer:
When a skier moves down a slope at a constant speed, the coefficient of friction between the skier's skis and the slope is such that it exactly balances the component of the weight of the skier parallel to the slope.
Step-by-step explanation:
When a skier moves down a slope at a constant speed, it means that the net force acting on the skier is zero. In this case, the frictional force opposing the skier's motion is equal to the component of the weight of the skier parallel to the slope.
Since the speed is low enough that air resistance can be ignored, the only force opposing the skier's motion is the frictional force. Therefore, we can say that the coefficient of friction between the skier's skis and the slope is such that it exactly balances the component of the weight of the skier parallel to the slope.