Externality

An apple farmer benefiting from a nearby beekeeper is a positive externality due to increased pollination (positive production). Hence, A corresponds to (III).
- Villages down stream from a manufacturing plant are affected by pollution, creating a negative externality (negative consumption). Hence, B corresponds to (II).
- An unvaccinated family benefiting from a vaccinated neighbor has positive consumption effects. Hence, C corresponds to (I).
The rice farmer near a chemical plant is harmed by negative production externalities (pollu tion). Hence, D corresponds to (IV).
Hence, the correct answer is (b)