Covered smut of Sorghum is caused by

The correct option is Sphacelotheca sorghi.

Explanation:
Smut diseases in sorghum are fungal infections caused by different species of the genus Sphacelotheca or related genera. Each species is associated with a specific type of smut symptom:
1. Sphacelotheca sorghi: This fungus causes Covered smut of sorghum (also known as grain smut). In this disease, the individual grains are replaced by oval or conical dirty-grey sacs containing black fungal spores (teliospores). The spore mass remains covered by a tough membrane (peridium) which usually remains intact until threshing.
2. Sphacelotheca reiliana: This pathogen is responsible for Head smut of sorghum, where the entire flower head (inflorescence) is replaced by a large smut gall.
3. Sphacelotheca cruenta: This organism causes Loose smut of sorghum, characterized by the rupture of the membrane early in development, exposing the loose black powder of spores.
4. Tolyposporium ehrenbergii: This is the causative agent of Long smut of sorghum.

Therefore, based on the specific disease characteristics, the covered smut of sorghum is caused by the fungus Sphacelotheca sorghi.