Concept:Assertive means confident and forceful in stating one's views and standing up for oneself.Solution:Mama's point is that bullies back off when a person refuses to be afraid and stands firm.So "assertive" here means confident and fearless, which is bold.A. "muscular" and B. "strong" describe physical power; being assertive is about attitude, not the body.C. "smart" means clever; a clever person may still be timid.Key point: Assertiveness is a quality of character (confidence), not of strength or intelligence.Answer:Option D (bold)