Answer:
b. A referendum
Step-by-step explanation:
The answer is referendum because it is a method in which all the people in a country that are qualified to vote are invited to do it on a particular subject and the result of this is the adoption or not of a new law.
The other options are not correct because an amendment is a change made to a law, an initiative is a petition to force the government to enact a law and a constitutional convention is a meeting to write a new constitution or to make changes to the one in place.