What does "mercurial" imply about a person?

The term "mercurial" describes a person who is subject to sudden and unpredictable changes in mood or behavior. This word derives from Mercury, the Roman god known for his swift movement and changeability, which perfectly encapsulates the essence of someone whose emotions or actions can shift rapidly and unexpectedly. Such individuals might fluctuate between enthusiasm and despondency or show variability in their responses to situations, making them difficult to predict.

In contrast, the other choices suggest traits that are fundamentally opposite to being mercurial. Stability and predictability, solemnity, seriousness, or being reserved and thoughtful indicate a steadfastness in disposition, which is not characteristic of someone who is mercurial. Thus, "liable to sudden unpredictable change" directly aligns with the core meaning of the term, making it the correct answer.