Why do light and dark patches appear in an otherwise uniform colour filling?

Correct answer: Due to uneven paint load and water quantity

The governing concept is uniform paint load and water-to-pigment ratio. When some strokes carry more pigment or more water than others, their value and transparency differ, creating light and dark patches. Thus option A is correct. A clean brush is helpful, while white paper and a closed shape do not by themselves create uneven tonal areas if the mixture and application remain consistent.