Answer: B. 
Step-by-step explanation:
We know that the radius of a circle is a line segment which starts from the center of the circle and ends to any point on the circle.
From the given figure , it can be seen that there are three line segments in the circle which starts from center to a point on circle .
Thus, The radius of the circle with center O= 
From the given options , line segment
is a radius of circle O.