Final answer:
Circular runout, when applied to a surface perpendicular to a datum axis, implies that the surface must be cylindrical, ensuring its elements are consistently equidistant from the datum axis for true rotation.
Step-by-step explanation:
The conditions that apply for circular runout when it is applied to a surface that is perpendicular to the datum axis relate to the tolerances and geometrical accuracy of the surface. The correct answer to the question is B) The surface must be cylindrical. Circular runout is a geometric tolerance that controls the circularity and coaxiality of a cylindrical surface relative to a datum axis. When applied to a perpendicular surface, this condition implies that the surface must align in a way that its elements, such as diameter and roundness, are consistently equidistant from the datum axis, ensuring that the part is true to its axis throughout rotation. Other answers like flatness or parallelism are different forms of geometric tolerances and do not directly relate to circular runout as it pertains to cylindrical surfaces.