What is the technique called where paint is applied very thickly on the canvas?

The technique where paint is applied very thickly on the canvas is known as impasto. This method involves layering paint so that it stands out from the surface, giving the artwork a textured appearance. Impasto allows artists to create dramatic effects with light and shadow, as the thick application of paint can catch and reflect light in varied ways. This technique has historically been used by many famous artists, such as Vincent van Gogh, to enhance the emotional intensity and dynamic quality of their works.

In contrast, the other options refer to different painting methods that do not emphasize thick paint application. Acrylic painting can be done in various styles, including thin washes, and does not necessarily imply thickness. Fresco is a painting technique involving water-based pigments applied on freshly laid wet plaster, which does not involve thick layers of paint. Watercolor, on the other hand, typically involves thin washes of translucent paint applied with water, resulting in delicate and fluid imagery, rather than the bold texture characteristic of impasto.