I must assume that your graph is that of a straight line, and that the end points of the line are P and B, and (finally) that T is between P and B. If these assumptions are correct, then the length of the line segment PB connecting points P and B is 15 + 10, or 25.
Use Pi, 3.14. Divide the Diameter by 3.14 and you should get your answer. That's all I know.
I’m not sure how to explain it but the answer is B