Answer :
The loan was for a six year period, this is 6 years * 12 months = 72 months.
He payed $225 per month for a total of $225 * 72 months = $16,200.
So in total he paid off $16,200 - $15,000 = $1,200 more than he loaned over a six year period. So the correct answer is: a. $1,200.