What Is the only constitutional amendment that was ratified by approval of conventions in three fourths of the states?

The only constitutional amendment ratified by approval of conventions in three-fourths of the states is the 21st Amendment. Ratified in 1933, it repealed the 18th Amendment, which had established Prohibition in the United States. The 21st Amendment was unique in that it was the first and only amendment to be ratified by state conventions rather than by state legislatures.