One part of a colour mixture looks different after drying although both parts appeared similar when wet. What may be the cause?

Correct answer: The layer thickness differed between the two parts

The governing concept is the change in perceived colour as paint dries. Different layer thicknesses can leave different amounts of pigment on the surface, so the thicker or more concentrated section may dry with greater depth or a different apparent value. Option B is therefore correct. Equal water, the same brush colour, and equal paper size do not account for a difference between two painted areas. Consistent layers improve uniformity.