How can depth be created in a sky using only one blue watercolour?

Correct answer: Use more pigment at the top and more water toward the bottom in a graded wash

The governing concept is value gradation. Even with one blue pigment, changing the pigment-to-water ratio produces a darker upper sky and a lighter area toward the horizon. The gradual transition creates an atmospheric sense of distance, so option A is correct. A uniform dark value removes depth, black lines do not model the open sky, and adding oil is unsuitable for a watercolour wash and changes the medium rather than creating controlled value.