Allocative efficiency is achieved when the quantity produced of a good is the one that society values most, meaning the benefit from one more unit equals the cost of producing that unit. This is the optimal quantity where marginal benefit equals marginal cost: producing more would add more cost than benefit, and producing less would leave additional value untapped. So the defining idea is producing the optimal quantity of some output. The other ideas describe producing at the lowest possible cost for a given output (productive efficiency), maximizing profits for a firm, or producing the maximum possible output, none of which capture the balance between value and cost that allocative efficiency requires.

Explicit costs are the actual cash outlays a firm makes to obtain resources for production. They involve money changing hands in the production process, such as paying wages, rent, or materials. Depreciation is not an explicit cost because it is a non-cash expense—an accounting allocation of the wear on capital rather than a current cash payment. The best description of an explicit cost is “payments that are actually made,” since it highlights the cash transactions that define explicit costs. Wages paid by a firm are also explicit costs, because they are payments that are made. Costs paid out of pocket convey the same idea, but the key is that explicit costs require real cash payments in the period.

The main idea is that clearly defined property rights give people a direct incentive to bargain over spillover effects, so externalities are internalized through private negotiation. When laws spell out who owns what and what rights each party has, those affected by a positive or negative externality can bargain to reach a deal that reflects the true costs and benefits of their actions. If transaction costs are low, the bargaining leads to an outcome where private decisions align with social costs and benefits, making the external effect part of the market price. So the incentive is to internalize externalities—to incorporate the external costs or benefits into private decisions through voluntary exchange. While clearer laws can also lower the costs of bargaining, that’s the enabling condition, not the main incentive.

The issue this question tests is how policymakers counter monopoly power to improve welfare. The standard ways are to increase competition or regulate the market. Breaking up a monopoly directly reduces market power by turning one strong firm into several, more competitive firms. Making it easier for new rivals to enter the market changes the competitive dynamics, preventing a single firm from keeping prices high. Regulating the market imposes rules on price or output to curb the monopolist’s ability to charge excessive prices, aiming to imitate competitive outcomes even when true competition isn’t present. Nationalizing the industry—putting it under government ownership—is not typically listed as a standard remedy in this context. While government ownership could eliminate private monopoly power, it replaces private control with a public monopoly and can bring different inefficiencies and political incentives. In basic discussions of remedies to monopoly, it isn’t treated as one of the usual policy options, which is why it’s not considered the correct choice here.