Which of the following set forms a group under multiplication:
Show Hint
Remember that the set of $n$-th roots of unity $U_n = \{e^{2k\pi i / n} \mid k = 0, 1, \dots, n-1\}$ always forms a finite abelian group under complex multiplication. For $n=4$, $U_4 = \{1, i, -1, -i\}$.