To solve this question, we need to understand the characteristics of each oligopoly model mentioned in the options:

• Cournot Oligopoly Model: In the Cournot model, each firm assumes that the output levels of its competitors are fixed and decides its own production output based on that assumption. This is a simultaneous decision-making process where firms do not change their decisions based on others before setting their quantity.
• Stackelberg Oligopoly Model: This model features a leader-follower framework. One firm sets its output first, and the other firms react to this decision. Hence, the decision is not simultaneous, but sequential.
• Chamberlin’s Oligopoly Model: This model is less commonly discussed but generally refers to monopolistic competition rather than oligopoly in the strictest sense.
• Bertrand Oligopoly Model: In the Bertrand model, firms compete on price rather than quantity, assuming that their competitors' prices are fixed before setting their own prices.

Given these descriptions, the model where each firm assumes its competitors' output remains fixed while it decides on its own output is clearly the Cournot oligopoly model.

Thus, the correct answer is the Cournot oligopoly model.