Which colour would be good for showing a calm sea?

Correct answer: Blue

Blue is a cool colour that commonly represents water, sky, depth, and calmness. A blue range, especially softer or lighter shades, can communicate the quiet surface and cool atmosphere of a peaceful sea. Yellow, red, and orange are warm colours and are more likely to suggest sunlight, heat, movement, or excitement. Therefore, blue is the most suitable answer, making option D correct.