What are Sunk Costs in Project Management?

The correct answer is 'D. has previously been incurred and cannot be changed.'

Sunk costs are expenses that have already been incurred and cannot be recovered. They are irrelevant to future decisions because they are not affected by any future actions. For example, if you invest $10,000 in a project and it fails, the $10,000 is a sunk cost. You cannot get that money back, so it should not be considered when making future decisions about the project.