The correct answer is CARTEL
A cartel arises in an oligopolistic market when the few firms that constitute it decide to set together the market price of their products, or the share of the total demand that will be attended by each producer, instead of competing, as this latter option would lead to lower firm profits.
Cartels are illegal, as they are harmful for consumers that will purchase the same product at a higher price, and for the development of technological innovations, as firms in a cartel do not have incentives for investing in technology, as they do not have to overcome competitors.