Collusion among firms is most likely to take place in c) oligopolies.
Oligopolies are characterized by a small number of firms that dominate the market. This limited competition allows firms to observe and react to each other’s pricing strategies and output levels. In such an environment, firms may find it beneficial to coordinate their actions rather than compete aggressively. This can lead to collusive practices where firms agree (either explicitly or implicitly) to set prices, limit production, or divide markets to maximize their collective profits.
In contrast, in perfect competition (a), numerous firms exist, making collusion virtually impossible due to the lack of coordination opportunities. Monopolistic competition (b), while having some degree of market power, also has many firms competing, which diminishes the likelihood of collusive arrangements. Meanwhile, monopolies (d) are single-firm markets, and there’s no need for collusion as there’s no competition to collude against. Thus, the oligopoly structure is the most conducive to collusion.