In the excerpt from Roll of Thunder, Hear My Cry, Cassie's motivation to keep quiet in spite of her anger is primarily related to not wanting to escalate the situation and possibly get hurt, not just by T.J. but also in the context of her own struggles and conflicts with Lillian Jean. Cassie recognizes that responding in anger could lead to further issues, and she feels the need to control her emotions to maintain some semblance of peace.
However, the most fitting option among the ones you provided would be: Cassie needs to keep her plan a secret. This implies that she is contemplating her actions and plans regarding Lillian Jean, and maintaining silence is crucial for her strategy.