The correct answer is: C. All rivals follow the oligopolist up to a certain price.
The kinked demand model is a model of oligopoly that assumes that firms will not change their prices if their rivals do not change their prices. This is because firms in an oligopoly are interdependent, meaning that the actions of one firm will have a significant impact on the other firms in the market. If one firm lowers its price, the other firms will likely follow suit, leading to a price war. However, if one firm raises its price, the other firms are unlikely to follow suit, as they will lose market share. This leads to a kink in the demand curve for the oligopolist, with a relatively flat demand curve for price increases and a relatively steep demand curve for price decreases.
Option A is incorrect because it assumes that all rivals charge the same price, which is not necessarily the case in an oligopoly. Option B is incorrect because it assumes that all rivals charge a price independent of the price charged by the oligopolist, which is also not necessarily the case. Option D is incorrect because it assumes that all oligopolists charge the price as independent sellers, which is not the case in an oligopoly, as firms are interdependent.