Two areas painted with the same watercolour mixture appear to have different values. What may be the cause?

Correct answer: The layer thickness differs between the areas

In transparent watercolour, value means the relative lightness or darkness of a colour. The same mixture can look darker where a thicker layer deposits more pigment and lighter where a thinner, more diluted layer is applied. Unequal brush loading, overlapping strokes, or different pressure can produce this difference. Therefore option A is correct; paper size, a clean brush, and a white palette do not by themselves change the painted value in this way.