Loose smut of wheat is caused by Ustilago nuda. It is a disease that can destroy a large proportion of a wheat crop. Loose smut replaces grain heads with smut, or masses of spores which infect the open flowers of healthy plants and grow into the seed, without showing any symptoms. It is a seed borne disease; infection occurs during Loose Smut flowering through wind-borne spores.
So, the correct answer is 'Loose smut of Wheat'.