What is the best definition for the word labile?

The word "labile" is most accurately defined as "fluctuating." This term is often used in scientific contexts, particularly in psychology and biology, to describe conditions or states that are unstable or prone to change. For example, a labile emotional state indicates feelings that can change rapidly and unpredictably. This aligns with the idea of variability and lack of stability inherent in the definition of "labile."

The other options do not capture this essence. "External" pertains to something outside of or outer, which does not convey the idea of change. "Meticulous" refers to showing great attention to detail, which is unrelated to the concept of fluctuation. "Integral" describes something that is essential or necessary, but again, this does not relate to the instability or change implied by "labile." Therefore, "fluctuating" stands out as the best definition for the term.