Step 1: Analyze the key terms. The question asks for a legal document that "compels attendance" under "penalty."
Step 2: Evaluate the options.
(1) Oath: A solemn promise to tell the truth, taken by a witness before testifying. It does not compel attendance.
(2) Conduct money: Money paid to a witness to cover their travel expenses for attending court. It facilitates attendance but does not legally compel it.
(3) Subpoena: A writ issued by a court to compel a person to appear and testify. Failure to comply can result in penalties, including fines or imprisonment. This perfectly matches the question's description.
(4) Perjury: The criminal offense of lying under oath. This is an act a witness might commit, not the document that calls them to court.
Therefore, the correct answer is Subpoena.