Which primary colour is commonly used to show sky and water?

Correct answer: Blue

The governing representational convention is that blue is commonly associated with natural features such as a clear sky and water. Hence option C is correct. Red and yellow are also traditional primary colours, but they are not the usual choices for these two examples. Orange is generally classified as a secondary colour in the traditional RYB model because it is produced by mixing red and yellow. Artists may vary colours for mood, but blue is the standard answer here.