Which of the following actions is unlawful under the Sherman Antitrust Act?

The Sherman Antitrust Act is a federal statute aimed at maintaining competition in the marketplace by prohibiting certain business activities that prevent competition. Standardized commission rates among real estate professionals can lead to price-fixing, which directly violates the principles set forth in the Sherman Antitrust Act. When businesses agree to set the same commission rates, they effectively eliminate competition, as consumers have no alternative pricing options, thus harming market dynamics.

The other actions listed do not relate specifically to antitrust issues. Discrimination based on familial status pertains to fair housing laws, not antitrust concerns. Selling your own home without an agent is a personal choice and does not involve collusion or anti-competitive practices. Lastly, making unsolicited calls to a restricted phone number may violate telemarketing regulations, but this does not pertain to competition in the marketplace as addressed by antitrust legislation. Therefore, the correct identification of standardized commission rates as unlawful under the Sherman Antitrust Act highlights the importance of competition and consumer choice in real estate and other industries.