What is the correct way to avoid an abrupt boundary between red-orange and yellow-orange in a sunset?

Correct answer: Create a controlled orange transition between them

The governing concept is gradual colour transition and blending. Red-orange and yellow-orange share orange as their common colour family, so a controlled intermediate orange can bridge them and soften the edge. The change should be gradual rather than an abrupt stripe. Blue-green would introduce an unnecessary contrasting area, black would darken both colours, and using one colour would remove the intended variation. Thus A is correct.