Answer:
Randomly select 100 people who voted in the last election.
Explanation:
Notice that the article is about the mean age of voters, that means they need to have a sample where there's no age restriction, or voter's preference on one candidate.
Basically, they need to randomly selected a certain number of people who just voted in the last election, because their population to write the article is just people who voted, then they will use that data set to find the average age of voters.
Therefore, the right answer is the third choice.