The correct answer is the last option - O Banquo, Banquo! Our loyal master's murder'd! This is the line that provides important information for the rising action of the play. This is what actually drives all subsequent events to play out - Macbeth started his rise to kingdom when he killed Banquo. Later on he moved on to kill the king himself - when he killed Duncan, he became the next king. But his murder of Banquo started it all - he kept seeing the ghost of Banquo everywhere he went, because his guilt was tearing him apart.
