What does the technique of sgraffito require for its execution?

The technique of sgraffito specifically involves scratching through a layer of wet paint to reveal a different color or a ground beneath. This method can create decorative patterns or images, adding texture and depth to artwork. When the top layer is still wet, the scratch marks create a contrast with the underlying layers, making the images or designs stand out prominently.

In the context of the other choices, a completely dry surface would hinder the ability to create the desired effect because it doesn't allow for the scratching through layers, and multiple colors or a series of thin washes do not inherently relate to the definition of sgraffito, which focuses primarily on the technique of scratching into wet media. Therefore, the correct choice emphasizes the necessity of having a layer of wet paint in the process.