What can be done to make blue colour appear lighter toward the lower sky?

Correct answer: Gradually lighten the colour toward the bottom

The governing concept is tonal gradation: changing the value of one colour gradually across a surface. To make the lower sky appear lighter, reduce the strength of the blue, use a lighter mixture, or blend toward a paler tone as you move downward. Option D describes this controlled transition. Adding black or using the same dark blue would make the lower area darker, while leaving it blank would break the painted sky rather than create a gradual effect.