The correct answer is: B. Marginal revenue curve always slopes downwards.
In a perfectly competitive market, firms are price-takers, meaning that they cannot influence the market price of their product. As a result, the marginal revenue curve for a perfectly competitive firm is always equal to the market price. The market price is determined by the intersection of the supply and demand curves for the product. The supply curve for a perfectly competitive market is horizontal, meaning that firms are willing to supply any quantity of the product at the market price. The demand curve for a perfectly competitive market is downward-sloping, meaning that consumers are willing to purchase less of the product at higher prices. The intersection of the supply and demand curves determines the market price and quantity.
The marginal revenue curve for a perfectly competitive firm is equal to the market price because the firm can sell any additional units of the product at the market price. As a result, the marginal revenue curve is always equal to the market price, which is always downward-sloping.
Option A is incorrect because the marginal revenue curve for a perfectly competitive firm always slopes downwards. Option C is incorrect because marginal revenue is not always equal to average revenue. Option D is incorrect because marginal revenue is not always less than average revenue.