Final answer:
The activity most likely to create a positive externality is when d) a person receives a flu shot.
Step-by-step explanation:
A positive externality is a benefit that is enjoyed by a third party as a result of an economic transaction. Out of the given options, the activity most likely to create a positive externality is d) a person receives a flu shot. When a person receives a flu shot, not only does it benefit their own health by reducing the likelihood of getting the flu, but it also helps to reduce the spread of the flu to others in the community. This spillover benefit is an example of a positive externality.