This is a tricky answer, because:
the length of the loan, the interest rate, and the down payment will all affect the total cost of the loan
--BUT--
your credit history can help determine the interest rate you are offered, the length of the loan offered, and the downpayment required.
I think this is an unfair question, but the answer the teacher is looking for is A. credit history because it has an INDIRECT effect on total cost and not a DIRECT effect like the others.