Final answer:
The scenario where California helps illegal aliens enter the state conflicts with federal immigration laws and is prohibited by the supremacy clause.
Step-by-step explanation:
The scenario that could not occur because of the supremacy clause of the Constitution is D. California passing a law that helps illegal aliens enter their state at the southern border with Mexico. This is because the scenario directly conflicts with federal immigration laws, which are a matter of national control according to the Constitution. The supremacy clause asserts that when state law and federal law clash, the federal law prevails, provided it is within the scope of the federal government's constitutional powers. Therefore, a state law that opposes federal immigration policy would not hold up under the supremacy clause.
The supremacy clause of the Constitution establishes that the Constitution and federal laws are the supreme law of the land. This means that if a state law conflicts with a federal law, the federal law prevails. Based on this, the scenario that could not occur because of the supremacy clause is D. California passing a law that helps illegal aliens enter their state at the southern border with Mexico. This is because federal immigration laws supersede any state laws regarding immigration.