Final answer:
Voters use an initiative to approve an amendment to their state constitution. This process allows citizens to bypass the legislature by putting new laws or constitutional amendments on the ballot for a public vote.
Step-by-step explanation:
The reform measure voters could use to approve an amendment to their state constitution is an initiative. An initiative is a process where citizens can propose a law or constitutional amendment and have it placed on the ballot to be voted on by the electorate. This allows voters to directly enact legislation or constitutional amendments, bypassing the state legislature. States like Michigan provide such a mechanism for their citizens, enabling them to actively participate in the state's legislative process.
A referendum, on the other hand, occurs when the state legislature passes a law or a series of constitutional amendments and presents them to the voters to ratify with a yes or no vote. Though similar to initiatives, referendums often originate from the legislature and are not typically citizen-initiated. These direct democracy tools allow voters to have a more significant impact on their state government and policies.