What is the correct method for making warm and cool greys of equal value?

Correct answer: First make identical greys then add tiny yellow-orange to one and tiny blue to the other

The governing concept is that value means lightness or darkness, while temperature describes a warm or cool bias. First mix two greys with the same black-white balance and check that their lightness matches. Then add a very small amount of yellow-orange to one and blue to the other. Option D is correct because it changes temperature while preserving nearly equal value; changing white or black separately would make the values unequal.