Which primary colour can create a calm and cool effect?

Correct answer: Blue

Blue is commonly used to suggest calmness, coolness, peace, and stability, so option D is correct. In colour theory, red and yellow are generally warm colours and often feel energetic or bright. Green may also seem calming, but in the traditional RYB model it is a secondary colour made by mixing yellow and blue, not one of the three primary colours. Thus blue best satisfies both conditions.