The correct answer is D. The C.G. of a semicircle is at a distance of r/2 from the centre.
The C.G. of a circle is at its centre. This is because the mass of the circle is evenly distributed around the centre, so the centre is the point where the net force on any object placed there is zero.
The C.G. of a triangle is at the intersection of its medians. This is because the medians of a triangle divide it into six smaller triangles with equal areas. The C.G. of each of these smaller triangles is at the centroid of the triangle, which is the point where the three medians intersect.
The C.G. of a rectangle is at the intersection of its diagonals. This is because the diagonals of a rectangle bisect each other, and the C.G. of a rectangle is located at the intersection of its two axes of symmetry.
The C.G. of a semicircle is at the centre of the semicircle. This is because the mass of the semicircle is evenly distributed around the centre, so the centre is the point where the net force on any object placed there is zero. However, the C.G. of a semicircle is not at a distance of r/2 from the centre. This is because the mass of the semicircle is not evenly distributed along the radius. The mass is concentrated at the edge of the semicircle, so the C.G. is closer to the edge than it is to the centre.