The average interest rate on 30-year, fixed-rate mortgages rose slightly to 6.88% this week, up from an eight-week low of 6.87% last week, Freddie Mac said. So far this year, rates have fluctuated between 6.74% at the end of January and 7.11% in mid-March. Fifteen-year mortgages, a popular option for refinancing, averaged 6.51% this week, up from an eight-week low of 6.47%. On one-year, adjustable-rate mortgages, lenders were asking an average initial rate of 5.56%, the same as last week's four-month low. The rates do not include add-on fees known as points.