How is 'Port' wine produced?

Port wine is produced primarily by adding brandy to the wine during fermentation, a process known as fortification. This method stops the fermentation process while retaining a significant amount of natural grape sugar, resulting in a sweet, rich wine with a higher alcohol content. The timing of adding brandy is crucial; it typically occurs when about half of the sugar has been converted into alcohol. This not only preserves some of the sweetness but also enhances the wine's complexity and longevity.

While other methods mentioned in the other choices might apply to different wine styles, they do not pertain to the production of Port. For instance, fermenting grapes without skin contact is typical of producing white wines, aging in clay jars aligns more closely with some traditional wine-making methods, and chilling with ice is not a recognized method for producing any style of wine, including Port. Thus, the fortification method is the defining technique that distinguishes Port from other wines.