When a firm has the opportunity to add a project that will utilize excess factory capacity (that is currently not being used), which costs should be used to help determine if the added project should be undertaken?

Incremental costs.

Explanation:

An incremental cost can be defined as the total cost (amount of money) that is being incurred by a company as a result of an additional unit that is is added to its production or project.

Generally, an incremental cost plays a significant role in the decision-making process of an organization.

Hence, when a firm has the opportunity to add a project that will utilize excess factory capacity (that is currently not being used), the costs which should be used to help determine if the added project should be undertaken is the incremental costs.

answercold,maybe nuclear war;