Which is most clearly a variable cost A rent B insurance premiums C salaries of employees under long-term contract D interest payments E wages of production workers?

E. Wages of production workers.

If a firm were to shut down production, rent would still be owed, insurance premiums would still have to be paid, they would have to honor their long term contracts, and interest payments would still have to be met. But it could lay off any workers not under contract.