Final answer:
The correct statement is that in Bertrand oligopoly each firm believes that its rivals will hold their output constant if it changes its output.
Step-by-step explanation:
In oligopoly, each firm believes that its rivals will hold their output constant if it changes its output. This is true in a Bertrand oligopoly. In a Cournot oligopoly, firms produce an identical product at a constant marginal cost and engage in price competition. In Bertrand oligopoly, firms produce identical products and set prices rather than quantities. Therefore, the correct answer is: in Bertrand oligopoly each firm believes that its rivals will hold their output constant if it changes its output.