Following are orders of some proper subgroup of group. Which subgroup is necessarily normal?
$a)5. $ $ b)2.$ $ c)3. $ $d)4. $
Here order of group is not given so how to decide which is the answer? I know if index of a subgroup is 2 or p if p is smallest prime dividing the order of group then that subgroup is normal.But to calculate index, we need order of group but it is not given.I don't know how to proceed further.