Which term would best describe behavior that changes quickly and unpredictably?

The term that best describes behavior that changes quickly and unpredictably is "volatile." In various contexts, such as psychology or finance, volatility refers to the degree of variation or instability. In psychology, it can denote emotional states that shift rapidly, while, in finance, it pertains to the rapid price changes in markets. This characteristic is marked by fluctuations that are often unexpected, reflecting a lack of stability.

The other options do not convey the same meaning. "Voluminous" refers to having a lot of volume or size, which does not pertain to unpredictability. "Vivacious" describes someone who is lively and spirited, focusing more on personality traits rather than the stability of behavior. "Voracious" indicates an insatiable appetite or eagerness, often related to eating or consuming, which again is unrelated to the concept of unpredictability in behaviors. Therefore, "volatile" is the only term that encapsulates the idea of rapid and uncertain changes in behavior.