Answer: Option 'D' is correct.
Explanation:
Since we know that
Incenter - It is the centre of the incircle of the triangle and it is the point of intersection of the angle bisectors of the triangle.
Circumcenter - It is the point of intersection of perpendicular bisectors of the triangle. It is also the center of circumcircle.
Orthocenter - It is the point of intersection of the altitudes.
Centroid - It is the point of intersection of the medians.
String should be attached to triangular shaped pieces of cardboard, so they will balance out the mobile at the centroid because medians divide the triangle into equal parts.
Hence, Option 'D' is correct.