The correct option is "I, II, and III" because monoclonal antibodies are indeed man-made proteins designed to target specific antigens, thereby stimulating an immune response to enhance the body's ability to combat infections and diseases. Their specificity makes them valuable in treating various conditions, including certain viral infections like the Nipah virus, where they can neutralize the pathogen or mark it for destruction by the immune system. This multifaceted role exemplifies their significance in modern therapeutics, as covered in clinical immunology and biotechnology materials relevant for the UPSC syllabus.