The correct answer is: "the price of money".
When an investment project is financed using a loan, the party which issues the loan gives an amount of money to a borrower in exchange for an interest payment, that has to be paid when the amount borrowed is returned. Therefore, the interest is the price of the money. It is the retribution earned by the lender in exchange for a temporary use of his money while, from the point of view of the borrower, it constitutes the payment made to get the funds he needs.
