In the United States and in many other countries, citizens have to register to vote "To <span>make sure people vote only once," since otherwise it would be very hard to track who has already voted. </span>
The correct answer is D) To make sure people vote only once.
In order to make elections fair and to ensure there is no fraud, voters need to register so that the vote counters can make sure they only vote once. This process is used at the local, state, and federal levels. This helps to eliminate fruad that took place in American elections, especially during the late 1800's and early 1900's.