The correct response among the options you provided regarding the power of the tribunes of the plebs in Roman government is:
They could veto any law or action of a magistrate.
Tribunes of the plebs were powerful because they had the authority to protect the interests of the plebeians (the common people) against the decisions of the patricians (the aristocratic class) and other magistrates. Their veto power allowed them to halt legislation and actions that they deemed harmful to the plebeians, which made them a significant force in the political landscape of Rome. This veto power made them guardians of the rights of the lower class and provided a direct check on the authority of the ruling elite.