Final answer:
The centerline in a drawing with coaxial diameters represents the center of the part, and a runout control is used to define the concentricity of those diameters.
Step-by-step explanation:
On a drawing that contains multiple coaxial diameters, the centerline represents the center of the part. This is crucial for ensuring that all the cylindrical features of the part are symmetrical about a common axis. When we talk about a runout control, this is used to define concentricity which refers to the tolerance that controls how much the median points of a feature's circular elements can deviate from a perfect, common center.
Therefore, the correct answer to the student's question would be option A) Center of the part; concentricity.