Which method can be useful for creating a neutral grey?

Correct answer: Mix black and white in a suitable ratio

Neutral grey is an achromatic colour, meaning it has little or no noticeable hue. The direct method is to mix black and white in a suitable ratio, so option D is correct. More white creates a lighter grey and more black creates a darker grey, while a balanced ratio approaches a middle neutral grey. Red with white makes pink, red with yellow makes orange, and blue with yellow makes green, so those distractors retain a hue.