What is the term for a person who exhibits quick and marked changes in mood or behavior?

The term that describes a person who exhibits quick and marked changes in mood or behavior is "volatile." When someone is referred to as volatile, it suggests that their emotional state can shift rapidly, often in response to their environment or internal feelings. This term is commonly used in psychology and interpersonal relations to highlight the unpredictability of a person's reactions.

In distinguishing this term from the others, "voluminous" relates to something large in volume or space and does not pertain to mood or behavior. "Voracious" describes someone who has an insatiable appetite, typically for food, but can also refer to a strong enthusiasm for a particular pursuit; again, this doesn't connect to changes in mood or behavior. "Xenophobia" is a term that denotes fear or hatred of strangers or foreigners, which is unrelated to mood fluctuations. Therefore, "volatile" is the most accurate choice for describing someone with rapid and significant mood changes.