The action that assigns actual costs to a project is Material withdrawal from stock (C). When materials are withdrawn from stock for use in a project, the actual costs associated with those materials are charged to the project, reflecting the consumption of resources. This is a critical step in project cost management, ensuring that all material costs are accurately accounted for and tracked against the project budget.

A Material availability check (A)is a step in the planning process to ensure materials are available for the project but does not itself result in cost assignment.

Saving a purchase order (B)initiates the procurement process but does not directly assign costs to the project until the goods or services are received and consumed.

Releasing a project budget (D)is the process of formally allocating funds for the project but does not directly result in the assignment of actual costs.