Which of the following actions is considered a violation of antitrust laws?

Colluding on property listings is considered a violation of antitrust laws because it restricts competition and manipulates market conditions. Antitrust laws are designed to promote fair competition and prevent practices that could lead to monopolies or unfair market control. When real estate agents or companies agree to fix prices, limit supply, or allocate markets among themselves, it undermines the competitive nature of the market, which can harm consumers by leading to higher prices and fewer choices.

In contrast, individual pricing strategies and independent agent commissions are generally lawful because they allow agents to set their prices without collusion. Group marketing initiatives can also be permissible as long as they do not involve any agreements that restrain trade or violate principles of healthy competition. By understanding this context, it's clear why collusion in property listings stands out as a direct infringement of antitrust regulations, which serve to safeguard fair market practices.