Answer:
b. grisaille
Explanation:
Grisaille is a technique of painting the whole piece in shades of grey or other neutral shades, usually as a form of underpainting for a painting done in oil. It is convenient because it shows where light parts are and transparent oil glazes can be put over it, to keep the tones.
One of the most famous usages of the technique is painting Odalisque in Grisaille by Jean Auguste Dominique Ingres. In this case, the artist did the grey underpainting, but could not finish the whole piece in time in order to display it. The art was then displayed in grey and remains like that until today.