Concept:The embryo of a bird develops from the germinal disc on the surface of the yolk.Explanation:In a fertilised bird egg, the germinal disc contains the developing embryo.This disc lies on the upper surface of the yolk.The yolk provides nutrients for the embryo's early development.The chalaza holds the yolk in position, albumen is the egg white, and the airspace is at the blunt end of the egg.Therefore, the embryo is located in the yolk.Answer:Yolk (Option B).