Which of the following wines is a fortified wine?

Fortified wine is one that has had extra alcohol added to it, typically in the form of distilled spirits, such as brandy. This process not only increases the alcohol content but also influences the wine's flavor profile and stability, enabling it to age longer.

Port, as a type of fortified wine, exemplifies this category. It is produced primarily in the Douro Valley of Portugal, and during fermentation, grape spirit is added to stop the fermentation process, which retains some of the natural sweetness of the grapes. This unique production method contributes to Port's rich flavors and higher alcohol content, typically around 20% ABV, making it distinct from regular table wines.

The other wines listed—Riesling, Chablis, and Sauvignon Blanc—are all examples of standard table wines and do not undergo fortification. These wines are made from fermented grape juices without the addition of extra alcohol, thereby having lower alcohol content, usually ranging from around 9% to 14% ABV.