Some cities finance their airports with a departure tax: every person leaving the city by plane is charged a small fixed dollar amount that is used to help pay for building and running the airport. The departure tax follows the a. benefits-received principle. b. ability-to-pay principle. c. flat-rate principle. d. public-choice principle.

To identify the correct principle, we must first understand what each principle entails: a. Benefits-received principle: A taxation principle based on the idea that individuals and businesses should pay taxes in proportion to the benefits they receive from the public goods and services provided by the government. b. Ability-to-pay principle: A taxation principle suggesting that people should pay taxes according to their ability to pay, with those who have more income and wealth paying more in taxes. c. Flat-rate principle: A taxation principle where everyone pays the same percentage of their income in taxes, regardless of their income level. d. Public-choice principle: A principle of taxation related to the efficiency of public spending, not a principle of taxation itself, so it doesn't apply here.

We need to determine which principle most closely applies to the departure tax scenario given in the exercise. a. Benefits-received principle: The departure tax is charged to individuals as they use the airport facilities, and the revenue is used to help finance the airport's construction and operation. Therefore, those who use the airport directly benefit from it and pay for the airport's maintenance and operation through the tax. This principle seems to align with the departure tax. b. Ability-to-pay principle: The departure tax is a fixed amount regardless of the individual's income or wealth, so it does not adhere to the ability-to-pay principle. c. Flat-rate principle: The departure tax is not based on a percentage of one's income, making it unrelated to the flat-rate taxation principle. d. Public-choice principle: As mentioned earlier, the public-choice principle is related to the efficiency of public spending and not a taxation principle.

Based on the comparison in Step 2, it is clear that the departure tax aligns with the benefits-received principle, as those who use the airport directly benefit from its services and contribute to its funding through the tax. Hence, the correct answer is: a. Benefits-received principle.