The correct answer is: C. sunk cost.
Sunk costs are costs that have already been incurred and cannot be recovered. They are irrelevant to future decisions, because they cannot be changed. For example, if a company has already spent \$100,000 on developing a new product, that \$100,000 is a sunk cost. The company should not consider that cost when deciding whether to launch the product, because it cannot get that money back no matter what decision it makes.
Marginal costing is a method of accounting that focuses on the costs that vary with the level of production. These costs are known as variable costs. Fixed costs, which do not vary with the level of production, are not included in marginal costing.
Historical costing is a method of accounting that records all costs incurred in a period, regardless of whether they are variable or fixed. This method is used to prepare financial statements.
Notional cost is a hypothetical cost that is used for planning or budgeting purposes. It is not an actual cost that has been incurred.
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