Concept:A locus is the set of all points satisfying a given condition.Here, the condition is equal distance from one fixed point.Explanation:Let the fixed point be O and the constant distance be r.Any point P on the locus must satisfy OP=r.Moving around O while keeping OP=r traces a round curve.This round curve is exactly a circle with centre O and radius r.Therefore, the locus of points equidistant from a fixed point is a circle.Answer:A. circle