The correct answer is: D. None of the above
The AR curve and industry demand curve are the same in case of perfect competition. In perfect competition, there are many firms in the market, each producing a homogeneous product. The firms are price-takers, meaning that they cannot influence the market price. The AR curve is the same as the demand curve for the firm, because the firm can sell all it wants at the market price.
In monopoly, there is only one firm in the market. The firm is the price-maker, meaning that it can influence the market price. The AR curve is downward-sloping, because the firm can charge a higher price for a smaller quantity.
In oligopoly, there are a few firms in the market. The firms are interdependent, meaning that their decisions affect each other. The AR curve is downward-sloping, but it is not as steep as the AR curve in monopoly. This is because the firms in oligopoly can compete on price, but they also have to consider the reactions of their rivals.