Final answer:
The example of air pollution from a factory with a plan to reduce emissions illustrates a negative externality wherein the factory's activities harm the environment without compensating affected parties. Reducing emissions is an appropriate measure to mitigate this externality.
Step-by-step explanation:
The best example of a negative externality and an appropriate plan for eliminating it is: b) Air pollution from a factory, reduce emissions. A negative externality occurs when an entity's actions cause harm to a third party, and there is no compensation for the affected parties. In this case, the factory's emissions lead to air pollution, which can have both environmental and health impacts on society.
To eliminate or reduce this negative externality, the factory could implement measures to reduce its emissions, such as adopting cleaner technologies or following stricter environmental regulations.