Which scenario best exemplifies an externality?

The scenario that exemplifies an externality is when a company pollutes a river affecting nearby fish populations. An externality occurs when a decision by an individual or a company has unintended consequences for third parties that are not directly involved in the transaction or activity. In this case, the company's decision to pollute the river is an action that has negative repercussions for the fish populations and the broader ecosystem, as well as potentially for local communities that rely on the river for fishing or recreation. This pollution is a classic example of a negative externality, where the costs of the company's actions spill over to others who are not part of the decision-making process.

The other scenarios do not illustrate externalities. The price adjustment by a producer to attract customers reflects standard market behavior and competition, while a consumer buying a product on sale is simply an individual transaction without broader impacts. Hiring additional workers to increase production is an internal decision within a firm that does not involve third-party effects in the same way that pollution does.