The optimal level of provision of ________ goods is where society's total willingness to pay per unit is equal to the marginal cost of producing the good.

A) public
B) private
C) all
D) normal

Public goods are non-excludable and non-rivalrous, meaning they are available to all members of society without additional cost for extra users. The optimal provision level is where the total willingness to pay equals the marginal cost of production, ensuring efficient allocation of resources.