The correct answer is option 'B', which is blue. Blue is the color commonly used to represent water. Here is an explanation of why blue is associated with water:

1. Perception of Color: Our perception of color is influenced by the way light interacts with objects. When light strikes an object, it absorbs some colors and reflects others. In the case of water, it absorbs light in the red part of the spectrum and reflects light in the blue part of the spectrum. This reflection is what gives water its bluish appearance.

2. Visual Representation: In various forms of visual representation, such as maps, diagrams, and illustrations, blue is often used to represent bodies of water. This convention helps to visually differentiate water from other elements or features.

3. Cultural Association: The association between water and the color blue is deeply rooted in culture and language. For example, phrases like "deep blue sea" and "ocean blue" are commonly used to describe bodies of water. Additionally, many languages have specific words for the color blue that originated from the perception of water.

4. Psychological Impact: Blue is known to have a calming and soothing effect on the human mind. This psychological association may also contribute to the choice of blue to represent water. Water is often associated with relaxation, tranquility, and serenity, and the color blue helps to reinforce these emotional connections.

In conclusion, the color blue is used to represent water due to the way light interacts with water molecules, the visual representation in maps and diagrams, the cultural association between water and the color blue, and the psychological impact of the color blue on our perception of water.