What type of costs should firms and individuals ignore when making decisions?

The answer is sunk costs also known as retrospective costs. It is a charge that has already been acquired and cannot be recovered. These are occasionally compared upcoming costs that may be acquired or changed if an act is taken which is called prospective costs. Both prospective and retrospective costs may be either fixed or variable costs. Though, many economists consider it a mistake to categorize sunk costs as "fixed" or "variable."