Which one of the following microbial agents is being commercially exploited as biocontrol agent

Answer: Option D

Trichoderma viride is the microbial agents that that is being commercially exploited as biocontrol agent.
Trichoderma viride is a fungus and a biofungicide. It is used for seed- and soil treatment for suppression of various diseases caused by fungal pathogens.
The fungicidal activity makes T. viride useful as a biological control against plant pathogenic fungi. It has been shown to provide protection against such pathogens as Rhizoctonia, Pythium and even Armillaria.