Which word best describes the opposite of "serene"?

The word that best describes the opposite of "serene" is "agitated." "Serene" conveys a sense of peacefulness and tranquility, indicating a calm and undisturbed state. In contrast, "agitated" denotes a state of disturbance or unrest, often characterized by feelings of anxiety, nervousness, or turmoil. This makes "agitated" a fitting antonym to "serene" as it captures the essence of being unsettled or troubled, which is in stark opposition to the calm and peaceful nature represented by "serene."

The other options—tranquil, calm, and composed—all maintain a sense of peace or composure, which aligns with the meaning of "serene" rather than opposing it. Therefore, they do not serve as effective antonyms.