5. the amendment is the only constitutional amendment that was ratified by approval of conventions in three-fourths of the states. a. fifteenth b. twenty-sixth c. eighteenth d. twenty-first

The correct option is D.
The 21st Amendment was the only amendment that was ratified by state ratifying convention instead of the state legislature. The ratification was done on Dec, 1933. When the 21st amendment was introduced, it completely repealed the 18th amendment. The Amendment was introduced as the result of the failure of 18th amendment, which prohibit the sale of alcoholic beverages.

I would say D . ^they are correct