The correct answer here is D, Julius Caesar. The Republic began officially in 509 B.C after the last Etruscan king was overthrown. In his stead, the patricians, the political and economical elite took control and from them was born the Senate, the official body who would lead Rome for more than 200 years. However, during the 1st century, and due to the impressive growth of Rome in both land and power, the senators began to fight with each other over control. This led to great corruption and while this happened Julius Caesar, a senator but most importantly a great military leader, tired of the corruption, led an army and invaded Rome itself against his main enemy, Pompey. Julius Caesar became victorious and declared himself dictator for life. It was then that Rome changed from a Republic to an Empire. Later on, after his murder, Julius Caesar was inherited by Augustus, his nephew and the Senate became a body that empowered the emperors. But the one who established the change was Julius Caesar. The others were the inheritors of what Julius Caesar had established, even if for only a short period before he was murdered.