A characteristic of an oligopoly is a. mutual interdependence in pricing decisions. b. easy market entry. c. both (a) and (b). d. neither (a) nor (b).

An oligopoly is a market structure where there are few large firms dominating the industry. It is characterized by having barriers to entry and unique products among competitors.

Now, let's evaluate each option given in the exercise: a. Mutual interdependence in pricing decisions: In an oligopoly, the actions of one firm often impact the actions of others, which means they are mutually interdependent. For example, if one firm lowers its prices, others may choose to do the same or respond by offering new products/features. This characteristic is true for oligopolies. b. Easy market entry: One of the main features of an oligopoly is the presence of barriers to entry. These barriers can include high setup costs, economies of scale favoring incumbent firms, or government regulations. As a result, it is not easy to enter an oligopoly market. This characteristic is false for oligopolies. c. Both (a) and (b): Since (b) is false, this option is eliminated. d. Neither (a) nor (b): As (a) is true, this option is eliminated.

Based on the evaluation of each option, the correct answer is: a. Mutual interdependence in pricing decisions.