Choice "D" is correct. The demand curve for any individual oligopolist is kinked sharply downward. This occurs because, in oligopoly market conditions, the other firms in the market will match any price reduction so they do not lose market share but will not match any price increase of an individual firm. Therefore, for the individual firm attempting to raise its prices beyond equilibrium, consumers will quickly buy from other firms in the market and demand will drop off sharply creating a kinked demand curve.
Choices "a", "c", and "b" are incorrect, per the above explanation.
