The correct answer is D.
A credit score (credit rating) is a number that reflects the likelihood of a person's ability to pay back a debt.
A credit score is used by money lenders and credit card companies to evaluate potential clients. They need to know the level of risk involved in the action of lending money to individuals, so that they don't lose all the money they lend. The higher the credit score, the better the chances of getting a loan.
A credit score is based in part on income and total debt. If a person applying for a loan is in debt already, his chances of getting the loan are lower, as the first debt has to be paid yet. Again, the higher the earnings, the better the possibilities to be approved for a loan, as more wages mean more disposable income to pay back the debt from.