The sum of the following series 1 + 6 + 9(1² + 2² + 3²)/7 + 12(1² + 2² + 3² + 4²)/9 + 15(1² + 2² + .... + 5²)/11+ ..... up to 15 terms is:
a. 7820
b. 7520
c. 7830
d. 7510