That's an intriguing problem! Let's solve it step by step.
A number is divisible by 9 if the sum of its digits is divisible by 9. Since M is a 2015-digit number, the maximum sum of its digits (A) would be 2015*9 = 18135.
The sum of the digits of A (B) would be 1+8+1+3+5 = 18.
Finally, the sum of the digits of B (C) would be 1+8 = 9.
So, the number C is 9. Isn't math fascinating?