Which word can best describe an action that is characterized by frequent changes?

The word "labile" is best used to describe an action characterized by frequent changes. In a scientific context, "labile" often refers to something that is unstable or prone to change, indicating variability and a lack of permanence. This term is commonly employed in various fields such as chemistry and psychology to denote systems or conditions that are dynamic rather than fixed.

In contrast, the other choices convey opposite meanings; "stable" denotes consistency and resilience against change, "static" implies a lack of movement or change, and "solid" suggests firmness and stability. Therefore, "labile" effectively captures the essence of frequent change, making it the most appropriate choice in this context.