Which of the following is illegal?

All of the options listed are indeed illegal under antitrust laws, which are designed to promote competition and prevent monopolistic practices in the marketplace.

Price fixing is an agreement between businesses to set prices at a certain level, which undermines free market competition and can harm consumers by leading to higher prices.

Bid rigging refers to an unlawful agreement among competitors to predetermine the winning bid for a contract, which disrupts competitive bidding processes and can result in inflated costs for goods and services.

Market allocation involves competitors agreeing to divide up markets or customers amongst themselves, preventing fair competition and limiting consumer choice.

Since all these practices are illegal and violate antitrust regulations, the option stating "all of the above" accurately encompasses the illegality of each individual item. Understanding the implications of these illegal practices is crucial for maintaining fair competition in business, ensuring that consumers have choices and fair prices.