Which of the following disease is caused by a fungus?

The correct option is C Black rust of wheat

Wheat leaf rust is a fungal disease that affects stems, leaves, and grains of wheat, barley, and rye. Puccinia triticina causes 'black rust', P. recondita causes 'brown rust', and P. striiformis causes 'yellow rust.' It is prevalent among all the wheat rust diseases, occurring in most wheat-growing regions.

So, the correct answer is 'Black rust of wheat.'