Final answer:
Tribunes were the Roman officials with the authority to veto actions of law courts, popular assemblies, and the Senate to protect the interests of the plebeians.
Step-by-step explanation:
The Roman officials that could veto the actions of Roman law courts, the popular assemblies, and the Senate were the tribunes. After 450 BCE, the plebeians met in a Plebeian Assembly that annually elected ten officials known as tribunes. These tribunes had the power to attend meetings of Rome's assemblies, the Senate, and the law courts. If they observed any action by a public body or official that could harm plebeians, they had the authority to say "Veto" or "I forbid" and halt that action. This veto power was a crucial mechanism by which the plebeians could defend themselves and balance the power of patrician officials. Tribunes could also introduce laws to the Plebeian Assembly for approval, further cementing their role as protectors of the plebeian interest.