The correct answer is black.
The sky is blue because of a phenomenon called Raleigh scattering. This scattering refers to the scattering of electromagnetic radiation (of which light is a form) by particles of a much smaller wavelength. Sunlight is scattered by the tiny molecules of air in Earth’s atmosphere. Blue light is scattered more than the other colors because it travels as shorter, smaller waves. This is why we see a blue sky most of the time.
If there were no atmosphere, there would be no molecules to scatter the sunlight. This means that all of the sunlight would reach the ground without being scattered, and the sky would appear black.
Option A, white, is incorrect because the sky is not white when there is no atmosphere. The sky is black when there is no atmosphere.
Option B, colorless, is incorrect because the sky is not colorless when there is no atmosphere. The sky is black when there is no atmosphere.
Option C, blue, is incorrect because the sky is not blue when there is no atmosphere. The sky is black when there is no atmosphere.