I think you meant to write
If so, then the answer is Choice A. {15, 16, 17, 18, ..., 50}
We simply list every natural number from 15 to 50, including both endpoints. The three dots mean "continue this pattern". This way we don't literally have to write out every single value (there are 50-15+1 = 36 values in this set).
Note: the set of natural numbers is {1,2,3,...} so a natural number is basically a whole number, but 0 is not included. Negative values aren't included either.