ANSWER
QS is a tangential line.
EXPLANATION
From the diagram, OS is a radius of the circle.
The radius can only be perpendicular to QS, if and only if QS touches the circle at, S.
The point S is called the point of tangency.
Since line QS is perpendicular to the radius, OS and QS touches the circle at only one point, it is called a tangent.
Therefore QS is a tangential line.
