Please see attached.
I've seen quite a few problems based on the concept being tested here, so will try to explain.
Given two vertices of a triangle, there are infine circles which has them as points on their circumference, all of them have centres along the perpendicular bisector of the two vertices. In this diagram since the centre is fixed along a line, there can be only one possible circle. Once this is established , the other parametes of the circle can be found.