The correct answer is: C. biological control of soil-borne plant pathogens.
Trichoderma harzianum is a fungus that is used as a biocontrol agent against a wide range of soil-borne plant pathogens. It is effective against both fungal and bacterial pathogens, and can be applied to soil or seed as a drench or spray. Trichoderma harzianum works by competing with pathogens for nutrients and space, and by producing metabolites that are toxic to pathogens. It can also colonize plant roots and protect them from infection by pathogens.
A. Reclamation of wastelands is the process of restoring land that has been degraded or abandoned to a productive state. Trichoderma harzianum is not a microorganism that is commonly used for this purpose.
B. Bioremediation of contaminated soils is the process of using microorganisms to clean up soil that has been contaminated with pollutants. Trichoderma harzianum can be used for this purpose, but it is not the most common microorganism used.
D. Gene transfer in higher plants is the process of transferring genes from one plant to another. Trichoderma harzianum is not a microorganism that is commonly used for this purpose.