MC = MR1 = MR2, this condition is used in

✅ Explanation: The condition MC = MR1 = MR2 refers to a scenario where a firm's marginal cost (MC) equals its marginal revenue (MR) in two different markets (MR1 and MR2). This is a hallmark of price discrimination, where a firm charges different prices to different customer groups to maximize its profits.

📌 Other Options Explanations:
-(b) Cartel: A cartel is a group of firms that collude to fix prices and output levels. While cartels can engage in price discrimination.
-(c) Monopoly: A monopoly is a market structure with a single seller. While a monopolist might practice price discrimination.
-(d) Monopolistic competition: A market structure with many firms selling differentiated products.