Which change can increase depth if a poster-colour sky looks flat and lifeless?

Correct answer: Keep it slightly darker above and lighter near the horizon

The governing concept is atmospheric value gradation. In a simple landscape treatment, a sky can read as deeper overhead and lighter toward the horizon; this gradual value change gives the surface spatial recession. Therefore B is correct. One uniform dark blue removes variation, black across the sky makes it heavy, and a red horizon introduces an unrelated colour shift rather than a controlled depth cue. The transition should remain smooth, not banded.