Firstly, Conditional past real mood is a statement where one explains what they would have done if some event had occurred, or what could happen, in other words, a theoretical action. The tense will be dependent on the action. Therefore, the only one that fits is C "If I had homework, I would have studied at the library." I properly states their theoretical scenario in which they had homework.
AS far as I know your credit score can't be affected by your parents. Your credit score can only be affected by the overdue payments and purchases you make.
