What effect is possible if too much white is added to lighten a dark colour?

Correct answer: The colour may appear pale and chalky

The governing concept is tinting: adding white raises the lightness of a colour, but excessive white reduces its saturation and strength. Thus option D is correct because the mixture may lose richness and look pale, opaque, or chalky. It does not automatically become transparent, darker, or warm; those effects are not guaranteed by adding white. White should be introduced gradually so the original hue remains controlled.