(d)
Since, $f(n) = \left\{ \begin{array}{r}
n^{2},\ \ \ \ \ \ \ \ if\ n\ odd \\
2n + 1,\ \ \ if\ n\ is\ even
\end{array} \right.\$
$f(1) = 1^{2} = 1\ \ \ \ \ \ \ \ f(2) = 2(2) + 1 = 5$
$f(3) = 3^{2} = 9\ \ \ \ \ \ \ \ f(4) = 2(4) + 1 = 9$
$\therefore\ \ \ \ f(3) = f(4)$
$\therefore f$ is not injective.
Also, $f$ is not surjective as every element of $N$ is not the image of any element of $N$