What is the main characteristic of a fresco in painting?

The main characteristic of a fresco in painting is that it is painted on plastered walls or ceilings. This technique involves applying pigment to freshly laid wet plaster, allowing the colors to become an integral part of the wall surface as it dries. This method not only provides a durable finish but also enhances the vibrancy of the colors used in the artwork. Frescoes are often found in historical contexts, decorating churches and public buildings, exemplifying significant artistic achievements of various cultures, particularly during the Renaissance.

In contrast, the other options represent different mediums and techniques that do not align with the defining feature of fresco painting. Using canvas as a medium is characteristic of easel painting; oil paints are primarily associated with canvas works, and a digital format involves modern technology that does not apply to traditional fresco techniques.