What is a circle group?
(a) a subgroup complex numbers having magnitude 1 of the group of nonzero complex elements
(b) a subgroup rational numbers having magnitude 2 of the group of real elements
(c) a subgroup irrational numbers having magnitude 2 of the group of nonzero complex elements
(d) a subgroup complex numbers having magnitude 1 of the group of whole numbers
I had been asked this question in a national level competition.
This question is from Groups in portion Groups of Discrete Mathematics