The correct answer is A. 18
Explanation
The basic requirements to register as a voter in the United States are: To be a US citizen, to be over 18 years of age and to comply with residency requirements by state. Additionally, it is necessary to clarify that each state has its requirements to vote. In XX century the minimum age required for voting was 21 years old, nevertheless, after the second world war, many people argue that if the young people could go to the war with 18 years old, they could vote too. So, the correct answer is A. 18.