Answer:
c. A group of firms with an explicit, formal agreement to fix prices and output shares in a particular market
Explanation:
Cartel is a group of few oligopolistic firms, who collude with each other to increase their joint profits & market domination.
So, they are a group of firms with an explicit, formal agreement to fix prices and output shares in a particular market
Example : OPEC (Organisation of petroleum exporting countries) is a Cartel.