Which of the following correctly describes the external benefit resulting from an individual's purchase of a winter flu shot? a. The flu shot reduces the likelihood others will catch the flu. b. The flu shot reduces the likellhood you will miss work as the result of sickness; therefore, you will earn more income. c. The flu shot is cheaper than the cost of treatment when you get the flu. d. The income of doctors increases when you get the flu shot.

The correct answer is: **a. The flu shot reduces the likelihood others will catch the flu.** **Explanation:** An **external benefit** (also known as a positive externality) occurs when an individual's action positively affects others who are not directly involved in the decision-making process. - **Option a** correctly identifies that when an individual gets a flu shot, it not only protects them from getting the flu but also reduces the chance of transmitting the virus to others. This broader community protection is a classic example of an external benefit. - **Option b** describes a **private benefit** to the individual who gets the flu shot, as it helps them avoid missing work and potentially earning more income. This does not directly benefit others. - **Option c** talks about the cost-effectiveness of getting a flu shot versus treatment, which is also a **private consideration** for the individual. - **Option d** mentions the increase in doctors' income, which is more of an economic ripple effect rather than a direct external benefit from the individual's action. **Therefore, option a is the correct description of an external benefit associated with getting a winter flu shot.** **Answer:** a. The flu shot reduces the likelihood others will catch the flu.