Potential buyer Sam wants to increase his credit score before applying for a mortgage loan. What factors should he consider that are normally included in the calculation of a credit score?

$($a$)$ Payment history, total amount owed, new credit, length of credit history

$($b$)$ Payment history, new credit, income, types of credit

$($c$)$ Marital status, types of credit, total amount owed, new credit

$($d$)$ Total amount owed, bank account balance, payment history, length of credit history