Which law or amendment to the constitution had the greatest impact on expanding voting rights between 1865 to today?

15th Amendment.

Explanation:

The 15th Amendment of the Constitution of America allowed all men to vote without racism and discrimination. The law after the Civil War granted the rights to the African Americans (now freed) to vote. Before ratifying the amendment, only white men in America were allowed to vote and elect the government. The 15th Amendment was rectified during the Reconstruction era along with other Amendments like the 13th and 14th.

c is the correct answer for this question.

i think it would be a