What clues helped Reynie discover Mr. Curtain's weakness?

Reynie’s discovery of Mr. Curtain's weakness is primarily rooted in his careful observation of Curtain's behavioral patterns and physical cues. Throughout their encounters, Reynie notices specific traits and habits that hint at Curtain's vulnerabilities. This attention to detail allows him to interpret Mr. Curtain's actions and reactions, revealing inconsistencies or signs of distress that indicate where Curtain might be less confident or powerful.

These behavioral patterns can include subtle reactions during moments of stress or the way he interacts with others, providing insight into his psychological state. In the context of the story, this understanding becomes crucial as it equips Reynie and his friends with the knowledge they need to confront and ultimately challenge Mr. Curtain effectively.

The other options, while they may contain elements that contribute to understanding Mr. Curtain, do not focus as distinctly on the observational skills that lead Reynie to recognize specific weaknesses in Curtain’s character. Interactions with others, relationships, and past challenges provide context but do not directly lead to the uncovering of Curtain's weaknesses in the same way that attentiveness to his behavioral patterns and physical cues does.