Calculate the prices for both bonds. Calculate the duration measure for both bonds as of now.
Higher duration means higher interest rate sensitivity.
Calculating the duration for a 15-year bond is tedious, and would not be asked on an exam. Again, Bond A has a higher interest rate risk, because of a higher duration. If all else remains the same, then the duration must decrease. We know that a longer time to maturity makes a bond more interest rate sensitive. If a year goes by, the time to maturity has decreased, therefor the sensitivity has gone down, which should be reflected in a lower duration.