A group boycott is an illegal antitrust violation under the Sherman Act. It occurs when two or more competitors conspire to refuse to do business with a third party, such as another competitor, supplier, or customer. The purpose of this concerted refusal to deal is to eliminate competition by isolating the target business from the market, thereby harming its ability to compete effectively. This directly matches the scenario of businesses conspiring against another business.

B. Market allocation: This is an illegal agreement between competitors to divide markets, territories, or customers, rather than conspiring against a specific business.

C. Price fixing: This involves competitors colluding to set prices for goods or services, not targeting a specific business through a boycott.

D. Tie-in arrangement: This is an agreement where a seller conditions the sale of one product on the buyer's purchase of another, separate product.

1. Federal Trade Commission (FTC). "Group Boycotts." Competition Guidance. This official government source defines a group boycott as an agreement among competitors not to do business with a targeted individual or company. (See Section: "Types of Violations," subsection on Group Boycotts).

2. U.S. Department of Justice (DOJ). "Antitrust Laws and You." This publication explains that agreements among competitors to boycott a supplier or a competitor are illegal. (See Section: "Illegal Business Practices," subsection on Group Boycotts).

3. Gaddy, G. & Hart, D. (2022). Maryland Real Estate Practice & Law (17th ed.). Dearborn Real Estate Education. Chapter 6, "Federal and State Laws Affecting Real Estate," discusses antitrust violations, defining a group boycott as two or more businesses conspiring against another business.

4. University of Maryland, Francis King Carey School of Law. Course: Business Associations. Course materials on antitrust law cover the Sherman Act, Section 1, which prohibits concerted actions like group boycotts that restrain trade. These materials distinguish boycotts from other violations like price-fixing and market allocation.