The amendment is the only constitutional amendment that was ratified by approval of conventions in three-fourths of the states.

a. eighteenth
b. twenty-sixth
c. fifteenth
d. twenty-first

The correct answer is letter D.
The 21st Amendment was the only constitutional amendment that was ratified by approval of conventions in three-fourths of the states. It had repealed the 18th Amendment of the US Constitution which was designed for the prohibition on alcohol during the 1920s. The 21st Amendment was ratified in 1933 and was a unique form of amendment that was repealed by state ratifying conventions.

One was to hold a leash on a despot and future despots to come, another was set to ensure no despots ever came to power.