Quiz: Recursive Structure

For the sum of digits problem, what is the recursive formula to compute the sum of digits of $n$?

1. A.sum(n) = (n mod 10) + sum(n / 10)
2. B.sum(n) = n + sum(n - 1)
3. C.sum(n) = sum(n - 10) + 10
4. D.sum(n) = n / 10 + n mod 10