In the Context of Free-Market Economics, the Use of Taxation

In the context of free-market economics, the use of taxation and/or regulation to compensate for or eliminate the social cost when the sale or consumption of products may benefit or harm other people who are not involved in the transaction and didn't pay for the product, falls under the assumption of:

A) many buyers and sellers.
B) absence of externalities.
C) presence of opportunity costs.
D) self-interest.
E) complete information.