I came across this problem that says:
If $Z(G)$ denotes the centre of the group $G$, then the order of the quotient group $G/Z(G)$ can not be which of the following?
(a) $15,$
(b) $25,$
(c) $6,$
(d) $4.$
Could someone point me in the right direction (a certain theorem or property that I have to use)? Thanks everyone in advance for your time.