Answer: Option(D) is correct.
Step-by-step explanation:
Correct option: Act as if they were monopolies.
Cartel is a group of few firms that work towards the common goal. Because all the cartel members works together, that's why they act like a monopolies.
They decided to produce at a level where their combined marginal revenue is equal to the combined marginal cost.
Because they behave like a monopolist, so they sell lesser quantity at a higher prices than would be in a perfectly competitive market.