The cost of an activity paid for by the individual and the third party is defined as

The cost of an activity paid for by the individual and the third party is defined as social cost.

The correct answer is internal cost. These are the most basic types of cost and are basically what you base your prices on. It's things like materials and labour costs and overhead costs and things like that. External costs don't fall within this domain because they cannot be influenced and determined by the individual and the third party.