Answer:
The correct answer is option d. 
Step-by-step explanation:
An oligopoly is a market structure where there are a few producers producing homogeneous products or similar products which are close substitutes. Because of a few firms, there is a high degree of competition in the market. 
The market decisions of a firm affect its rivals, so all the firms are interdependent on each other. 
The firms are price makers. There is high restrictions on entry of firms in the market.