What should be done to make dark blue in watercolour?

Correct answer: Use less water and more paint

The governing concept is pigment concentration in a watercolour wash. Using less water with more blue pigment makes the mixture more concentrated, so it produces a darker value on the paper. Therefore, option B is correct. A large amount of water dilutes the pigment and creates a lighter wash, leaving paper unpainted creates no blue, and adding oil is not an appropriate watercolour method.