What is the technique of applying a thick layer of acrylic colour called?

Correct answer: Impasto

The governing painting-technique concept is impasto, in which paint is applied in a thick, raised layer so that brush or knife marks and surface texture remain visible. Thus option A is correct. A wash uses diluted paint in a thin, usually transparent layer; glazing also refers to a thin transparent layer placed over a dry layer. Dry-brush application uses relatively little paint to create broken texture, not a deliberately thick raised coating. The defining clue is thickness and relief.