What prevents a state from raising the legal voting age?
a. most state constitutions
b. the declaration of independence
c. executive order #2504
d. the 26th amendment to the constitution

Correct  

The 26th Amendment lowered the voting age from 21 to 18.

Context/details:

There were strong protest movements going on in America during the 1960s, leading up to 1971 when the 26th Amendment was passed by Congress and ratified by the states.  Protests against the Vietnam War were active on college campuses across the country, with communication between students and media reports being a big part of the story.  Part of those protests were the demands of young people chanting, "Old enough to fight, old enough to vote."  Since 18 was old enough to be drafted to fight in the war, young people argued for their right to vote as full citizens.

The 26th Amendment was passed by Congress on March 23, 1971.  It was ratified by the states faster than any other amendment, achieving ratification by July 1, 1971.

The 26th Amendment is worded as follows:  

b) race riots in several cities