Answer: Registered voters
Step-by-step explanation:
The National Popular vote proposal is one that guarantees that the president is one who will get the most votes from all of the United States. It works by states pledging their electoral votes to the person who wins the popular vote.
Of the options listed, the only group that opponents could argue would not see their interests represented are the registered voters because they alone can vote.
Under-age people are not allowed to vote and neither are immigrants regardless of their legal status because they are not American citizens and criminals are unable to vote as their voting rights are suspended.