Which factor would make a bank think that a prospective buyer would be more likely to pay a mortgage on time?

Mark him the smartest.

The best answer is "The buyer paid over 20% of the sales price of the home as a down payment." This indicates that not only is the buyer paying above the minimum down payment, which means he/she has more cash than the minimum needed to purchase the home, but also that the monthly payments, interest, and total loan will be lower. All of this indicates a less risky prospective borrower.