Which term refers to the cost of one unit change, considering the impact on society?

B. marginal social

The cost of one unit change refers to the additional cost incurred when one more unit of a good or service is produced or consumed. In economics, this is known as the marginal cost. When considering the impact on society, it is called marginal social cost.

• Average social cost is the total social cost divided by the number of units.
• Total social cost is the sum of all costs (private and external) for all units.

Therefore, the cost associated with a one-unit change is the marginal cost.

The final answer is $\text{B}$.