Final answer:
The acronym for the chief professional organization for psychologists in the United States is APA, which stands for the American Psychological Association.
Step-by-step explanation:
The acronym for the chief professional organization for psychologists in the United States is C. APA, which stands for the American Psychological Association. The APA is the largest such organization worldwide and includes a diverse range of members, from students to doctoral-level psychologists, involved in many different areas such as clinical psychology, education, research, and health. As a hub for the profession, the APA plays a crucial role in advancing psychological knowledge, accrediting university programs, publishing scientific journals, and representing psychologists at a national level.