Which federal law prohibits monopolies and promotes fair competition?

The Sherman Anti-Trust Act, enacted in 1890, is a federal law that prohibits monopolies and promotes fair competition. This act was created to prevent businesses from engaging in anti-competitive practices, such as price-fixing and monopolistic behavior. The other options listed are all unrelated to the question of which federal law prohibits monopolies and promotes fair competition. Option A is about a broker receiving commission, option B is about an agent acting without permission, and option C is about a court preventing a principal from denying agency. None of these choices pertain to anti-trust laws or fair competition. Therefore, the most accurate and relevant answer is D.