Explain the ways that one interaction from act 4 between a major character and a minor character developed the plot of the tragedy of julius caesar.

The audience learns about the trouble within the Second Triumvirate, the breakdown of Brutus and Cassius' friendship, and Octavius' troops being strong and ready to fight Brutus' army.
This is the only option which tells us something about minor characters and their stories, which obviously develop the plot of this play. Brutus decided to kill Caesar not because he was envious of his power, but because he thought he might become a tyrant - he did it in order to protect Rome and its citizens, which makes him the only 'positive' character in the play. However, his interactions with other minor characters move the plot even after Caesar was murdered.