Which of the following practices is used to extract color from grape skins during fermentation?

Punching down is a vital practice in winemaking that is used to extract color, flavor, and tannins from grape skins during fermentation. This process involves pushing the floating grape skins, known as the cap, back down into the juice. Since the skins contain pigments (anthocyanins) that contribute to the wine's color, this technique enhances the extraction of these compounds.

Moreover, punching down also ensures that the fermentation process is uniform and helps in managing temperature; it can prevent the buildup of carbon dioxide and aid in the yeast's performance. This physical manipulation of the grape skins significantly impacts the final wine's sensory properties, notably its color and structure.

In contrast, the other practices listed serve different purposes. Racking typically involves transferring wine from one vessel to another to clarify it and remove sediment. Cold stabilization is a process used to prevent tartrate crystals from forming in the finished wine, and filtration is commonly used to clarify the wine and remove microorganisms or particulates post-fermentation. None of these focus specifically on the extraction of color from grape skins during fermentation.