Answer:
It is called d. the Coase theorem
Explanation:
"If private parties ... over externalities on their own" is the common wording of the Coase theorem.
Besides the commonly mentioned requirement that transaction cost must be zero, there are other assumptions to be satisfied including:
1. Clearly defined property rights (to bargain on)
2. No wealth effects (because the money you receive/pay for the benefits/costs will make you poorer/richer and change how valuable they are to you)