The answer is:
C. Because of Caesar's ambition
In the excerpt Brutus finishes with this line:
"and death for his ambition. "
As in the book they were alla little jealous of what Caesar had accomplished and wanted to kill him because they believed he would meant the ruin for the empire, people loved Caesar and wanted him emperor, but his pairs didn´t like him and decided to betray him, Brutus and the others needed a good reason to justify their actions, Caesar´s ambition and pride were the perfect flaws to justify its murder.