Which Amendment was the most important for Voting Rights?

The Fifteenth Amendment (Amendment XV) to the United States Constitution prohibits the federal government and each state from denying a citizen the right to vote based on that citizen's "race, color, or previous condition of servitude."

according to classical republican political theory, which democratic republics would function best?

a.   small republics, with people who had similar interests

b.   large republics, with people who had competing interests

c.   republics made up entirely of landowners

d.   republics made up entirely of servants

is there multiple choice or do you need like an essay answer?