Marcus Junius Brutus is most well-known for the assassination of Julius on the Ides of March, so he would be the best answer. Marcus Aurelius lived many years after the death of Julius, so he would not be a good choice. Augustus Caesar was Julius's adopted son and successor, so he too would not be a good choice. Pompey the Great was an ally of Julius Caesar before he crossed the Rubicon, so he would not be a good choice either. And finally Romulus is the mythological founder of Rome, and if he did live it was centuries before Julius, so he would not be a good choice.

The First Triumvirate of Rome was an unofficial political alliance between three prominent Roman noblemen, Julius Caesar, Pompey the Great, and Marcus Crassus, all of whom were seeking personal advancement and gain. The actions of the First Triumvirate contributed to the downfall of the Roman Republic and the emergence of the Roman Empire.

Tiberius and Gaius Gracchus were both populares in the latter years of the Roman Republic. This means they relied on the support of the common people to acquire political power. They both served as plebeian tribunes and led failed attempts at political and class-based reform in the Roman Republic.

Gaius Marius is an extremely important figure in the history of the Roman Republic. He is perhaps most notable for the reforms he enacted to professionalize the Roman army. He was also the primary rival to Lucius Sulla. The two competed for control of the city of Rome during the First Civil War of the Roman Republic.

Ptolemy XIII was one of the last rulers of Hellenistic (Greek) Egypt. He is most notable, in a study of Roman history, for organizing the murder of Pompey the Great when the latter was unfortunate enough to seek refuge in Egypt. Ptolemy thought that murdering Pompey would please Julius Caesar (who was Pompey’s main rival for power), but it actually greatly offended Caesar who ordered that Pompey be brought back to Rome for a proper burial.

During the history of the Roman Republic, not including the exceptional rise to power of Julius Caesar, dictators were usually appointed by the Roman Senate to lead the Republic during times of crisis or warfare.

Julius Caesar crossed the Rubicon in 49 BCE, in open opposition of the Senate, and by doing so essentially declared war on them, so he would be the best choice. Augustus Caesar was Julius's adopted son, and while he was a general in his own right he did not cross the Rubicon in a famous manner, so he would not be a good choice. Marc Antony was an ally and then an enemy of Augustus, but he did not cross the Rubicon with Julius, so he too would not be a good choice. Romulus is the mythological founder of Rome and Remus is his brother, so neither of them would be good choices.

Julius Caesar, Pompey the Great, and Marcus Crassus were the members of the original Triumvirate. The alliance grew out of a mutually beneficial arrangement between Caesar and Pompey wherein both agreed to block legislation or actions by the Senate that would hurt either one. Constantine the Great, however, lived centuries after the First Triumvirate and was himself a Roman emperor, so he was clearly not alive just before the establishment of the Empire. Therefore, Constantine the Great would be the best answer.

Julius Caesar was infamously murdered on the floor of the Senate by a group of renowned Roman noblemen. Caesar had recently had himself declared dictator for life (an unprecedented move in the history of the Roman Republic) and the conspirators feared Caesar’s immense popularity with the Roman people. In an attempt to preserve the Roman Republic, and their own personal power, the conspirators murdered Julius Caesar.

All of these men in some way or another contributed to the downfall of the Roman Republic and the emergence of the Roman Empire, but it was Octavian (later called Augustus Caesar) who would eventually consolidate power and have himself crowned as the first emperor of the Roman Empire.