Marginal revenue equals marginal cost at $6 for the 5th unit produced. Total revenue equals total cost at both the 4th and 5th unit, but this is not the determining factor.

Trashing your room really only affects you. Only littering, which affects the general public, is clearly a negative externality.

Among the available choices, only poorly defined property rights is likely to result in an externality. For example, poorly defined intellectual property rights could result in firms not reaping the full benefit of their research and development and therefore doing less than is socially optimal.

Although few real-life markets meet the characteristics of perfect competition exactly, the lack of perfect (or even equal) information about the goods being traded stands out in the used car market. When dealing in used cars, the seller typically has much more information about the quality of the car than the buyer.

The other answers more or less fit the perfect competition model. There are many buyers and sellers, the property rights of the buyer and seller are well defined, there are limited returns to scale (i.e. small used car sellers have not been competed out of existence), and factors of production (labor, land) can be put to other uses.

The total cost of producing 7 units is $49. Since the total cost of producing 6 units is $38, the marginal cost of producing the 7th unit must be $11.

The law of diminishing marginal utility states that the marginal value derived from a unit decreases as its use increases. This explains the phenomenon where the use of capital (its marginal product) decreases or diminishes as its utilization increases.

A public good is both non-rival (a person's use does not prevent another person's use) and non-rivalrous (a person cannot be excluded from using it). The only good that fits this description is the online blog. 

A market or industry that is dominated by a small number of firms is called an oligopoly. By contrast, monopolies are dominated by one firm, duopolies are dominated by two firms, and perfectly competitive and monopolistically competitive industries have many firms competing with one another. 

An increase in the demand for pizza will increase its price and quantity, while a decrease in the supply of pizza will increase its price and decrease its quantity. Thus, the price of pizza will unambiguously increase, but the quantity can either increase or decrease, depending on which change has the greater effect on supply.

When two countries trade two goods with one another, they will trade the goods that they have a comparative advantage in producing. One nation can have an absolute advantage in the production of both goods, but each nation will still have a comparative advantage in producing one of the two goods.