Final answer:
The Internal Revenue Code section holds the highest tax validity as it is statutory law, making it the most authoritative source among the options provided.
Step-by-step explanation:
The question is asking about the hierarchy of tax authority, specifically which source has the highest tax validity. The sources mentioned include regulations, revenue rulings, Internal Revenue Code (IRC) sections, and revenue procedures. Among these, an Internal Revenue Code section holds the highest authority because it is statutory law passed by Congress. Therefore, the correct answer to the question is c. Internal Revenue Code section. Regulations, while having significant authority, are still a level below the IRC as they are Treasury Department interpretations of the IRC. Revenue rulings and procedures provide guidance on the IRS's interpretation of the law but do not carry the weight of the IRC.