A commonly used biocontrol agent against plant fungal diseases is:

Trichoderma species act as antifungal biocontrol agents. (1) Bacillus thuringiensis (Bt): Used as a biopesticide to control insect larvae, not fungal diseases. (2) Rhizobium: A nitrogen-fixing bacterium that forms symbiotic relationships with leguminous plants, not a biocontrol agent. (3) Trichoderma: A fungal biocontrol agent that suppresses plant pathogenic fungi like Fusarium, Pythium, and Rhizoctonia. It promotes plant growth and enhances resistance against diseases. (4) Agrobacterium: A genetic engineering tool used to transfer genes into plants, not used for fungal control. Thus, the correct answer is option 3.