I'm pretty sure the answer should be 2.

Each digit has an assigned value, then these values are added together.

0 = 1
1 = 0
2 = 0
3 = 0
4 isn't used
5 = 0
6 = 1
7 = 0
8 = 2
9 = 1

So 2581 would be 0 + 0 + 2 + 0 = 2.