One key difference between an oligopoly market and a competitive market is that:

a) each firm's profits depend on other firms actions in oligopolistic markets while they do not in competitive markets.

b) oligopotisfic firms sell completely unrelated products while competitive firms do not.

c) oligopolistic firms sell their product at a price equal to marginal cost while competitive firms do not.

d) oligopolistic firms are price takers while competitive firms are not.

The correct option is a) each firm's profits depend on other firms actions in oligopolistic markets while they do not in competitive markets.

Oligopolistic firms are interdependent. This means that the action taken by a firm will directly affect the profits of other firms in the industry. On the other hand, competitive markets are characterized by many firms that are independent. This means that the action of a firm will not directly affect the industry due to the low market concentration in the market.

It is important to remark that both types of markets sell related products. Further, oligopolies sell at a higher price than the marginal cost and competitive markets sell at a price equal to marginal cost. Finally, oligopolistic firms are price makers and competitive firms are price takers. Therefore, options b), c) and d) are wrong.