Natural numbers are: 1, 2, 3, 4, 5, 6, 7, 8, 9, ........ so on
So, the first five natural numbers are: 1, 2, 3, 4, and 5
Sum of these five natural numbers = 1 + 2 + 3 + 4 + 5 = 15
Additive identity is the number which when added to the number, the number remains unchanged.
Therefore the additive identity for 15 is nothing but '0' because when 0 is added to 15, the number 15 remains unchanged.