Final answer:
The tribunes, elected by the Plebeian Assembly, had the power to veto actions, thus checking the executive power within the Roman Republic government.
Step-by-step explanation:
The part of the Roman Republic government that checked on the executive power was the tribunes. The tribunes were officials elected by the Plebeian Assembly and they had the power to veto actions of not only the Roman law courts and the popular assemblies but also of the Senate itself. If they observed any public body or official, including the consuls, taking action that could harm the plebeians, they could intervene by saying 'Veto' or 'I forbid' to stop that action. This significant veto power served as a check to protect the rights of the plebeians and balance the power of the patrician officials within the Republic government.