Final answer:
The American Academy of Professional Coders (AAPC) is the organization that primarily focuses on certifying medical billers and coders.
Step-by-step explanation:
The organization that focuses primarily on certifying medical billers and coders is the American Academy of Professional Coders (AAPC). The AAPC is a professional association that provides certification programs for medical coders and billers. They offer certifications such as Certified Professional Coder (CPC) and Certified Professional Biller (CPB).
Other organizations mentioned in the options, such as AMT (American Medical Technologists), NCCT (National Center for Competency Testing), and NHA (National Healthcareer Association), also offer certifications in the healthcare field but their focus is not solely on medical billing and coding.