Here's why:

• i. MR (Marginal Revenue): In a perfectly competitive market, the marginal revenue is constant and equal to the market price. Therefore, the MR curve is a horizontal line.
• ii. AR (Average Revenue): Average revenue is calculated as total revenue divided by quantity. Since the price is constant, the average revenue is also constant and equal to the market price. Therefore, the AR curve is a horizontal line.
• iii. Price line: This is a direct representation of the market price, which is constant for a price-taking firm. Therefore, it's a horizontal line.
• iv. Demand curve: For an individual firm in a perfectly competitive market, the demand curve is perfectly elastic, meaning it can sell any quantity at the prevailing market price. This is represented by a horizontal line.

Therefore, all four options are correct.