"Oligopoly is the only market structure in which rivalry among firms takes place." Do you agree or disagree? Why?

What will be an ideal response?

Agree. Oligopolists recognize their strategic dependence and must take into consideration likely reactions by rivals to any decision they make. The firms realize they are in competition with other firms and may actively compete with them. Under perfect competition and monopolistic competition, there is no rivalry because the firms are too small to have any impact on the other firms. Each firm acts independent of the other firms. By definition there is no rival under monopoly. So, oligopoly is the only market structure where competitive behavior actually takes place.