The answer is A: Julius Caesar.
Explanation for each option:
A. Julius Caesar - Correct. Julius Caesar was a pivotal figure in the transition from the Roman Republic to the Roman Empire. His crossing of the Rubicon River in 49 BCE led to a civil war, and his subsequent dictatorship laid the groundwork for the end of the Republic. His assassination in 44 BCE was a key event that eventually led to the rise of his heir, Augustus (Octavian), who became the first Roman Emperor.
B. Pompey - Incorrect. While Pompey was a significant military and political leader during the late Republic, he was ultimately defeated by Julius Caesar in the civil war. His death marked the decline of the Republic's power structure, but he did not play a direct role in the transition to the Empire.
C. Brutus - Incorrect. Brutus is best known for his role in the assassination of Julius Caesar. Although he was involved in the events that followed Caesar's death, he did not contribute to the establishment of the Roman Empire.
D. Cicero - Incorrect. Cicero was a prominent statesman, orator, and writer during the late Republic. He was an advocate for the Republic and opposed the rise of dictatorial power, but he did not play a direct role in the transition to the Empire. His influence waned after Caesar's assassination, and he was eventually executed during the power struggles that followed.