Final answer:
Voters could use an initiative as a reform measure to change a law about taxes.
Step-by-step explanation:
The reform measure that voters could use if they wanted to change a law about taxes is Option 2: An initiative. An initiative is a citizen-initiated petition process to either enact a law or make a change to a state constitution. This allows citizens to directly propose and advocate for changes in tax laws by collecting signatures from voters and presenting the measure on the ballot for all eligible voters to decide.