The correct answer is The Tenure of Kings and Magistrates
Das Kapital was written by Karl Marx while the Prince was written by Niccolò Machiavelli. On Kingship was written by Thomas Aquinas, which leaves only the Tenure of Kings and Magistrates as the piece of work written by Milton. He wrote it as a guide for rulers because of his experience as a government official and wanted others to understand what is helpful and what isn't.