Concept:A locus is the set (path) of all points that satisfy a given condition.Solution:The condition here: every point is the same distance from one fixed point.All such points lie on a circle: the fixed point is the centre and the fixed distance is the radius.A. A chord is a line segment joining two points on a circle, not a set of equidistant points.C. A mediator (perpendicular bisector) is the locus of points equidistant from two fixed points.D. A diameter is a chord that passes through the centre.Key point: Equidistant from one point → circle; equidistant from two points → perpendicular bisector (mediator).Answer:Option B (circle.)