The free-living fungus Trichoderma can be used for:

Trichoderma is a free-living fungus that is used effectively for plant disease management. It reduces or stops the pathogen attacks by various mechanisms such as enzyme secretion, competitive inhibition, and antibiosis. Trichoderma doesn’t kill insects or control butterfly caterpillars and they do not produce antibiotics.

Thus, option b. biological control of plant diseases is the correct answer.