The correct answer is c) cartel.
A cartel is an agreement between competing firms to control prices or production with the aim of maximizing their joint profits, essentially behaving like a monopoly. Unlike a natural monopoly, which occurs when a single firm can supply the entire market more efficiently than multiple competing ones, a cartel consists of multiple firms collaborating rather than competing. This formed alliance often leads to higher prices and reduced output, similar to the practices of a monopoly. Other options such as increasing cost industry, perfect competition, and nationalized industry do not fit the description of a group of firms acting together like a monopoly.