Answer:
a. an oligopoly.
Explanation:
Oligopoly - it is that form of structure in which no firm can keep another firm to have significant influence. Monopoly refers to one firm, the duopoly is referred to two firms while oligopoly refers to the firm that is more than 2 numbers.
example of an oligopoly is automobile manufacturing - there are many automobile manufacturing unit and they are continuous in competition and not allowing others to capture the whole market.