Which technique is suitable for showing dark blue at the top of a sky and light blue near the horizon?

Correct answer: A dark-to-light graded wash

The governing technique is a graded wash, in which one colour changes gradually in value or strength across a surface. Applying deeper blue near the top and progressively reducing its intensity toward the horizon creates the requested atmospheric transition. Option A is therefore correct. Dots create texture rather than a continuous sky, a black boundary does not create tonal change, and one uniform colour cannot show the dark-to-light shift.