Which of these describes $f : \mathbb{N} \to \mathbb{N}$, $f(n) = n^{2}$?

Explanation

1. 01

   On $\mathbb{N}$ there are no negatives, so $a^{2} = b^{2}$ gives $a = b$: one-one.

2. 02

   2 is not a square: not onto.