The answer is C, use a debit card, or use a credit card and pay the balance in full once the bill arrives. A debit card acts like a cash card directly from your bank account therefor there is no interest when you use the card. A credit card can also be used as if it were cash on hand (if in your mind you treat it that way). You can make the purchase and you have until your statement arrives to pay it off. If you pay it in full before the due date, no interest will be applied.