Final answer:
The MR curve in a Perfectly Competitive market is flat because marginal revenue equals the market price. In contrast, the MR curve in a monopoly market slopes downward because to increase sales, a monopolist must lower the price, thus affecting all units sold.
Step-by-step explanation:
The way Marginal Revenue (MR) lines on a Perfectly Competitive (PC) graph and on a Monopoly graph differ relates to the slope of these curves. In a PC market, the MR curve is flat because each additional unit of the good sells at the market price; hence, the additional revenue from selling one more unit (marginal revenue) is constant and equal to the market price. This can be represented as (b) PC MR curve is flat, Monopoly MR curve slopes downward, indicating the correct answer to the question.
Conversely, the MR curve in a monopoly market slopes downward. This is because a monopolist faces a market demand curve rather than taking the price as given, meaning that to sell additional units, the monopolist must lower the price. This lowering of price affects not only the additional units sold but also all the units that could have been sold at a higher price, which is why the MR curve lies beneath the market demand curve for a monopolist.