Final answer:
The largest single group of alternative-complementary therapy practitioners in the United States is not explicitly identified, but the field includes various types of practitioners and forms part of a broader health care system that embraces medical pluralism.
Step-by-step explanation:
The single largest group of alternative-complementary therapy practitioners in the United States is not clearly identified in the given information. However, the healthcare system in the US does include various practitioners of alternative medicine such as those specializing in Chinese medicine, Ayurvedic medicine, homeopathic medicine, and chiropractic medicine.
The diversity of medical practices, often referred to as medical pluralism, indicates a blend of biomedicine with ethnomedicinal systems. It's also worth noting the historical perspective from the provided references, which suggests that societal changes and movements influenced the rise of different therapeutic practices, with psychologists at a conference in Swampscott, Massachusetts in 1965 calling for a discipline that responds to social change.