No act can be considered a federal crime unless it is?

A federal crime is defined as an act that is prohibited by federal law. This means that the act must be explicitly outlined and banned by statutes or regulations that are enacted by the federal government. If an action is not specifically prohibited by federal law, it cannot be classified as a federal crime, regardless of its nature or context.

The other options provided do not accurately capture this definition. While common law and the Assimilative Crimes Act may influence the interpretation of certain crimes, they are not prerequisites for an act to be classified as a federal crime. Additionally, not all federal crimes need to be felonies; there are also federal misdemeanors. Therefore, the correct understanding hinges on the fact that for an act to be labeled a federal crime, it must be explicitly prohibited by federal law.