Answer: B. The firm would install the filter at a cost of $ 300,000.
Explanation:
If the community owns the property rights, they would be able to demand that the firm pay the external cost of $500,000 per year.
If on the other hand the company installed a filter, it would cost them $300,000 but then they would not have to pay the community the $500,000.
The lower cost option would be to install the filter for $300,000 which is what the firm would do.