according to wiki,”a tone is produced either by mixing a color with grey, or by both tinting and shading” so it’d probably be d but honestly cannot tell if that’s supposed to grey or blue
There are multiple colors used for a landscape is wasn’t to spec if but if it’s shading you need to use a shade darker than the one you originally painted with and if it’s just a regular tone witch is what the question is asking the tone should be white
