The answer is option C: Fifteenth amendment
The 15th Amendment, ratified in 1870, is very well known for granting African American men the right to vote for the first time, thus the right to vote was expanded to other American citizens.
It is stated in Article XV, Section 1 and 2.
"Section 1. The right of citizens of the United States to vote shall not be denied or abridged by the United States or by any State on account of race, color, or previous condition of servitude.
Section 2. The Congress shall have the power to enforce this article by appropriate legislation."