Question

Which of the following sets is equal to {1,2,3...}?

Answer 1

{x | x ∈ N, x ≥ 1} means: 
X where X is in the set (∈) of (N)atural number (counting numbers, 1, 2, 3, 4, etc) where X is greater than or equal to 1. This is your guy. 
The R is for real numbers, and that would be 1, all the numbers between 1 and 2 (like 1.33 repeating, the square root of 3, 1.7, etc), 2, all the numbers between 2 and 3, 3, all the numbers between 3 and 4