Final answer:
When two firms limit production and raise prices without a formal agreement, it is known as tacit collusion, occurring in oligopoly market settings to maintain higher profits. Option d is correct.
Step-by-step explanation:
If two firms limit production and raise prices, even though they had not made any formal agreement, then this is an example of tacit collusion. Tacit collusion occurs when businesses in an oligopoly market situation informally agree to charge higher prices and limit production, resulting in a form of silent cooperation where they can share monopoly-level profits.
This differs from explicit collusion where there is a formal agreement, such as in a cartel, to restrict outputs and set prices. In this scenario, the firms are using a mutual understanding to avoid competitive pricing which would otherwise benefit consumers.