Final answer:
The government can prompt a producer to internalize pollution costs by implementing pollution taxes, which incentivize firms to reduce pollution and choose the social optimum.
Step-by-step explanation:
A way the government can prompt a producer to internalize pollution costs that were previously external is by implementing pollution taxes. With pollution taxes, firms face an additional cost for polluting goods, which incentivizes them to reduce pollution and choose the social optimum. For example, a carbon tax is a type of pollution tax that directly taxes carbon dioxide. This approach encourages firms with lower costs of abatement to undertake pollution control, while those with higher costs can continue to pollute and pay the tax.