In a light-to-dark gradation, the middle area suddenly became too dark. Which error is most likely?

Correct answer: The amount of dark colour increased suddenly in the middle

The governing concept is gradual tonal transition: a light-to-dark gradation should change smoothly, not in abrupt bands. If too much dark pigment is added in the middle, the tonal value jumps suddenly and creates a dark patch. Therefore, option C is correct. Smoothly increasing pigment or pressure would prevent the error; paper smoothness, applying light first, or brush size alone does not explain the sudden dark area.