Step 1: Identify the context — Siti is relaxing by a lake, suggesting a calm and quiet atmosphere. Step 2: Analyse the target word — 'tranquil' means calm, peaceful, and free from disturbance. Step 3: Match to the synonym — 'peaceful' captures the same meaning of calm and undisturbed. The other options (noisy, anxious, furious) all carry negative or contrasting connotations.