The correct answer is (a). The black pottery of Thanjavur is known for its metallic sheen. It is made from a special type of clay that is found in the region and is fired in a kiln at a high temperature. This gives the pottery its distinctive black color and metallic sheen.
Option (b) is incorrect because the black pottery of Thanjavur is not known for its engraved designs.
Option (c) is incorrect because the black pottery of Thanjavur is not known for its vibrant colors.
Option (d) is incorrect because the black pottery of Thanjavur is not known for its religious motifs.