Which antitrust practice involves competitors agreeing to set their commissions?

Price fixing occurs when competitors in the same industry agree to set their prices, including commissions, at a predetermined level. This practice is illegal as it disrupts the natural competition in the marketplace, which ultimately harms consumers by keeping prices artificially high. When competitors collude to control pricing, it eliminates the incentive to compete based on service quality or better pricing, leading to less innovation and fewer choices for consumers. Therefore, the act of agreeing to set commissions among competitors falls squarely within the definition of price fixing, making it the correct answer.

In contrast, group boycotts involve competitors agreeing not to deal with a particular business, tying arrangements refer to conditioning the sale of one product on the purchase of another, and market allocation entails dividing markets among competitors to avoid competition in those designated areas or segments. While these practices are also anti-competitive, they do not specifically pertain to the agreement on commissions in the same way that price fixing does.