B is not a radius because a radius is a segment from the center of the circle to another point on the circle. In this case the radius is the dotted red line in the picture.
B is not a diameter because a diameter is a segment that passes through the center of the circle and its endpoints lie on the circle. In this case the diameter is the dotted black line in the picture.
B is not the center because the center of a circle is a point in the middle of the circle. B is a curved line and not a point. In this case the center is the point where the radius and the diameter meet.
This means that B must be a great circle based on process of elimination.
