Which combination is most suitable for producing a smooth colour gradation with acrylic paint?

Correct answer: Damp surface and a medium that controls open working time

The governing concept is extending acrylic paint’s workable, or open, time so adjacent colours can merge before drying. A slightly damp working surface and an appropriate medium slow premature drying and support a gradual transition. Therefore option C is correct. A dry canvas, a rigid knife, varnish, or loose pigment cannot provide controlled blending; a thick dry layer usually creates hard edges instead.