Final answer:
The argument that most clearly supports the 26th Amendment is that eighteen-year-olds should have the right to vote if they are old enough to be required to fight for their country.
Step-by-step explanation:
The argument that most clearly supports the 26th Amendment is option D: Eighteen-year-olds can be required to fight for their country in Vietnam, so they should also have the right to vote. This argument is based on the idea that if young adults are old enough to be drafted into the military and potentially risk their lives for their country, they should also have a say in the political decisions that can directly impact them. The 26th Amendment was ratified in 1971 in response to the activism of young people who argued for the right to vote.