The correct answer is: D. a-1, b-2, c-3, d-4
An indifference curve is a graph that shows all the combinations of goods that a consumer is indifferent between. It is downward-sloping because a consumer will always prefer to have more of both goods, rather than less.
A demand curve is a graph that shows the relationship between the price of a good and the quantity demanded of that good. It is downward-sloping because consumers are willing to buy more of a good when the price is lower.
Perfect competition is a market structure in which there are many buyers and sellers of a homogeneous good, and no one buyer or seller has any market power. In perfect competition, the price of the good is determined by the market, and firms are price-takers.
Oligopoly is a market structure in which there are a few large firms that sell a homogeneous or differentiated good. In oligopoly, firms have some market power, and they can influence the price of the good.
Price leadership is a market structure in which one firm sets the price for the good, and the other firms follow that price. Price leadership is most common in oligopoly markets.
In conclusion, the correct answer is: D. a-1, b-2, c-3, d-4.