Which mixture is most suitable for creating a sky-blue colour?

Correct answer: Add enough white to blue

The governing concept is tinting. A tint is formed when white is added to a colour, reducing its strength and increasing its lightness. Adding enough white to blue therefore produces a light sky-blue shade, so option A is correct. Black would darken blue, red with yellow tends toward orange, and red mixed with green usually produces a dull neutral or brownish colour.