After Tarquin, the Romans set up a republic because:

a) all power should rest in the hands of people

Incomplete options but the people didn't want a monarchy rule anymore.

It happened because the last king of Rome, Lucius Tarquinius Superbus, had a son who committed a grievous act which led the people to send the king and his family in exile. This led to the establishment of the Roman Republic.

The people were able to select the leaders to rule them yearly afterwards and to have a say and greater impact in the government.