Which one of the following methods was used only once to ratify an amendment? a. by approval of conventions in three-fourths of the states b. by approval of the supreme court c. by approval of three-fourths of the state legislatures d. by approval of the president

The correct answer is A. By approval of conventions in three-fourths of the states

The most common way is the third one, three-fourths of state legislatures and a supermajority of the congress approves it, however, only once the first method was used. It is a legitimate option outlined in the constitution. The congress can call for national conventions which can pass the amendment but this doesn't happen often.