One key characteristic that is distinctive of an oligopoly market is that

The correct answer is: interdependence of firms.

Explanation:

One distinctive feature of an oligopoly market is the interdependence of firms in the market. In an oligopoly market, there are few firms. These firms may produce homogenous or differentiated products.

The firms are price makers and face a downward-sloping demand curve. Because of few firms, there is a high degree of competition in the market. This makes the firms interdependent on each other.

If a firm takes some decision regarding price or output it will affect its competitors, so the firms have to consider the reaction of its competitors before making a decision.