Which word denotes unpredictability in someone's behavior?

The word "erratic" is indeed the term that denotes unpredictability in someone's behavior. When someone is described as erratic, it implies that their actions or responses are inconsistent and can vary widely without warning. This quality of unpredictability can manifest in various ways, such as mood swings, sudden changes in opinion, or behavior that does not align with established patterns.

In contrast, the other terms have distinct meanings that do not convey the same sense of unpredictability. "Strident," for example, refers to a loud and forceful sound or an aggressive way of expressing opinions, suggesting intensity rather than unpredictability. "Cynical" describes a belief that people are motivated purely by self-interest, which doesn't imply unpredictability but rather a skeptical view of human nature. Finally, "vengeful" conveys a desire for revenge or a tendency to seek retribution, pointing to deliberate and often predictable behavior driven by emotion rather than unpredictability. Thus, "erratic" stands out as the ideal choice to represent unpredictable behavior.