The correct answer is Group X has had its voting rights denied; group Z has had its rights abridged.
When a person is prevented from voting, it means that their right to vote was denied. This was the case with Group X.
On the other hand, Group Z's right to vote was abridged. This is due to the fact that they had to present their utility bill as a means of verification. Without the utility bill, members of Group Z could not vote.