What is required for a subpoena to be valid?

For a subpoena to be valid, it must be signed by a judge. This requirement underscores the legal authority behind a subpoena, which is a document that compels an individual to appear in court or produce documents and records. The signature of a judge signifies that the court has reviewed the request and has found sufficient legal grounds to issue the subpoena, giving it its enforceable power.

The other options do not fulfill the legal criteria necessary for a subpoena's validity. Approval from a compliance officer, general notification to employees, and authorization from the Office of Inspector General (OIG) may be relevant in various contexts but do not substitute for the judicial oversight required for a valid subpoena. The judicial signature is essential to ensure that the subpoena is issued through proper legal proceedings, thus protecting the rights of individuals and enforcing due process.