Which mixture is correct for creating sky blue with poster colour?

Correct answer: Add enough white to blue

The governing concept is tinting: a tint is made by mixing a colour with white. Sky blue is a light tint of blue, so adding white gradually to blue produces the required lighter shade. Black would make the blue darker, while the other mixtures would create dull or neutral results rather than sky blue. The exact shade depends on the proportion, so white should be added little by little.