Final answer:
Regulations are the administrative source of primary authority among the options provided, with the U.S. Constitution and court rulings being other forms of primary authority. The correct options are c.
Step-by-step explanation:
Among the options provided, c. regulations is an administrative source of primary authority. Administrative sources come from agencies tasked with interpreting statutes and implementing legislation. The U.S. Constitution is a foundational legal document, providing the framework for the federal government and is the supreme law of the land.
Collected rulings of various courts on Federal tax matters would be considered case law or judicial sources of primary authority. Newsletters would not be considered a primary or authoritative source in any legal context as they are informal, often non-official communications.