We are told that, for an individual firm in a perfectly competitive market, its demand curve is a horizontal line. What does this imply?

This firm's marginal revenue is always going to equal to its current price, no matter how many units it sells.

The consumers for this firm are very abnormal: they are not going to respond to any changes in the firm's price!

This individual firm is free to choose the price it is going to charge.

This individual firm is forced to sell a positive quantity, and is not allowed to exit the market at any point in time.