Final answer:
Gesso is a preparatory substance made from white pigment, chalk, and a binder used in painting as a ground. The term 'medium' refers to the binding agent in painting and is the correct answer to the student's question.
Step-by-step explanation:
The student is asking about a basic substance used in painting, specifically in preparing surfaces like canvas for painting, which is known as gesso.
Gesso is a substance traditionally made from a combination of white pigment, chalk, and a binder. It's applied as a ground to create a non-porous layer between the canvas (or other support) and the paint layer, preventing the paint from soaking into the support and causing potential damage over time.
Modern gesso often uses acrylic polymers and latex. For the question provided, the correct answer would be D) Medium.
'Medium' in the context of painting refers to both the materials used to create artwork as well as the substance that holds together the pigment and allows it to adhere to the surface.