Concept:Smut disease is a plant infection caused by fungal pathogens.Explanation:Smuts are caused by fungi belonging to the class Ustilaginomycetes.These fungi produce large numbers of dark teliospores on cereal grains.Fungi are eukaryotic organisms that grow as multicellular threads and absorb nutrients from the host plant.Among the options, the agent that matches this description is a fungus.Answer:B. fungus