Answer: third option.
Step-by-step explanation:
A tangent of a circle is defined as a line that touches that circle in exactly one point.
You can observe in the figure attached that:
The segment MN and the segment ST are not the tangent of the given circle, because this segments intersect it in two points.
The segment RO is the radius of the circle.
The segment PQ touches the circle in one point.
Therefore, the segment PQ is the tangent of the circle.
