What is the correct way to make light blue in watercolour?

Correct answer: Add more water to blue

Watercolour is a transparent medium, so its value is controlled mainly by the pigment-to-water ratio and the white paper beneath it. Adding more clean water to blue spreads the pigment into a paler, lighter wash, making option B correct. Black would darken the colour, red would shift it toward violet, and yellow would shift it toward green rather than simply creating light blue.