Which doctrine states that federal courts must follow state law?

The Erie Doctrine is the principle stating that federal courts must apply state law when deciding cases under diversity jurisdiction. This doctrine originated from the Supreme Court case Erie Railroad Co. v. Tompkins in 1938, which addressed the issue of whether federal courts should adhere to state laws or allow federal common law to govern. The ruling emphasized that in matters of state law, federal courts should defer to the appropriate state's legal standards to ensure consistency and fairness in the judicial system.

The significance of the Erie Doctrine lies in its role in maintaining the balance between state and federal judicial responsibilities, particularly in situations where the legal issues at stake are grounded in state law. This means that when a case is brought in federal court but is based on state law due to the parties being from different states, the federal court must apply the relevant state law as if it were a state court.

The other options, while relevant to various legal principles, do not specifically address the requirement for federal courts to follow state law like the Erie Doctrine does. This makes the Erie Doctrine the correct choice for understanding federal court procedures in relation to state law.