The free living fungus Trichoderma can be used for which of the following purpose ?

Biological control of plant diseases.

The free-living fungus Trichoderma has been widely used for the biological control of plant diseases. Trichoderma species are known for their antagonistic properties against various plant pathogens, including fungi, bacteria, and nematodes. They have the ability to colonize plant roots and suppress the growth and activity of pathogenic microorganisms through mechanisms such as competition for nutrients, antibiosis (production of antibiotics), mycoparasitism (attacking and parasitizing other fungi), and induction of systemic resistance in plants.

Trichoderma-based biocontrol agents are applied to crops and soils to prevent or manage plant diseases, reduce the use of synthetic pesticides, and promote sustainable agriculture. They can be used as seed treatments, soil inoculants, or foliar sprays to enhance plant health and protect crops from various fungal and bacterial pathogens.

While Trichoderma-based products might indirectly contribute to controlling certain insects or arthropods by targeting their associated plant pathogens, their primary use and effectiveness lie in the biological control of plant diseases, rather than directly killing insects or arthropods.

Therefore, the correct purpose for which Trichoderma can be used is "Biological control of plant diseases."