If a state official wished to challenge the constitutionality of a federal law, which amendment would she or he most likely cite?

The Tenth amendment

Explanation:

Tenth Amendment was part of the bill of rights. It states that the powers which are not specifically delegated to the federal government by the constitution belongs to the people and state. Its purpose is to define the division of power between state and Federal government. It was used to define the federal taxing power, federal regulations and police power. It got added to the Constitution due to the determination of Anti - Federalists.

So if the state official wants to challenge a federal law, he would cite the tenth amendment because it specifies the state and federal powers.