The roots of nth roots of unity are
$1, \omega,\omega^2.........\omega^{n-1}$ and $\omega^n=1$
$G= \{1, \omega,\omega^2,........ \omega^{n-1}\}$ is a group with respect to multiplication .
Identify element is 1.
$\omega^k. \omega^{n-k} =\omega^{k+n-k}=\omega^n =1$
The inverse of $\omega^k$ is $\omega^{n-k}$
Hence 3 is the correct answer.