Final answer:
The conspirators decide not to involve Cicero in their plan because he is not trustworthy and won't keep their secret.
Step-by-step explanation:
The conspirators decide not to discuss their plan with Cicero because D) he is not trustworthy and won't keep their secret. In ancient Rome, Cicero was a well-known politician and orator, but he was also notorious for being verbose and not being able to keep secrets. The conspirators feared that if they involved Cicero in their plan to assassinate Caesar, he would eventually reveal the plot under pressure or inadvertently through his conversations and speeches. Therefore, they chose to exclude him from their discussions.