This is a piece of cloth used by artists for painting on, usually with oil paints. What is it?

The correct answer is canvas. Let's look at why the other options are incorrect: * Frame: A frame is a border used to hold and display a finished painting. It's not the surface you paint on. * Brush: A brush is a tool used to apply paint to the surface. * Sketch: A sketch is a preliminary drawing or outline. A canvas is a specially prepared piece of cloth (usually linen or cotton) stretched over a frame, which is specifically designed to be painted on, especially with oil paints.