What is the benefit of building a dark area with several watercolour layers?

Correct answer: It gives greater control over value and colour temperature

The governing concept is gradual layering, often called glazing when transparent washes are built one over another after drying. Each layer increases the dark value step by step while allowing the artist to adjust warmth, coolness, and transparency. Thus option C is correct. A single heavy application is harder to correct and may reduce luminosity; layering does not make paint immediately opaque, keep paper permanently dry, or eliminate the need for a brush.