Final answer:
The action that is explicitly prohibited under the Sherman Anti-Trust Act is C. Price fixing, as it directly restricts competition in the marketplace by setting prices artificially.
Step-by-step explanation:
The action that is prohibited under the Sherman Anti-Trust Act is C. Price fixing. The Sherman Anti-Trust Act is a fundamental statute in the United States antitrust law that was passed in 1890 with the purpose of preventing anti-competitive practices. This federal law prohibits activities that restrict interstate commerce and competition in the marketplace, which includes price fixing, bid rigging, and dividing markets by allocating customers, suppliers, territories, or lines of commerce. While referral fees and dual agency are practices that might raise concerns under specific circumstances, it is price fixing that is explicitly prohibited under the Sherman Anti-Trust Act.