Final answer:
Explicit collusion, such as cartels, which involves formal agreements between firms, are more likely to face legal challenges since they provide clear evidence of anti-competitive practices that violate antitrust laws.
Step-by-step explanation:
When considering types of collusion between companies that are likely to face legal challenges from competitors or regulatory bodies, it is the explicit collusion that typically stands out. Explicit collusion, such as cartels, involves formal agreements between firms to control prices, limit production, or divide markets, which are direct violations of antitrust laws.
On the other hand, tacit collusion occurs when firms implicitly reach an understanding that competition is bad for profits without any explicit agreement. Although tacit collusion can be challenging to prove, explicit collusion presents hard evidence that regulatory bodies like the Antitrust Division of the Justice Department and the Federal Trade Commission can act upon.
Cartels are an example of explicit collusion, and because they offer clear proof of anti-competitive behavior, they are more likely to be legally challenged and are rare in the United States.
The correct option in this case is b. Explicit.