Based on the excerpt from Julius Caesar, the most appropriate response is:
Brutus realizes that arguing with Cassius is unimportant.
This conclusion is supported by Brutus's reaction to Cassius's anger and his own admission of grief over Portia's death. Instead of continuing the conflict with Cassius, he chooses to let go of their unkindness and focus on more important matters, highlighting his emotional state and prioritizing their friendship.