Answer:
d. Many firms are working together to eliminate pollution
Explanation:
Coase theorem is a private solution for the two parties who agree to reduce externalities, i.e., pollution. They negotiate in such a manner that the costs are low as one party takes over other party's polluted assets to reduce pollution. When there are more parties or firms involved to eliminate pollution, it will pose high transaction costs. Therefore, the Coase theorem will not work in that case. So, the option "D" is the correct choice.