What is a common characteristic of fortified wines?

Fortified wines are distinguished by the addition of spirits during or after fermentation, which increases their alcohol content. This process not only raises the alcohol level but also impacts the flavor, sweetness, and stability of the wine. For example, ports and sherries are well-known types of fortified wines that have been enriched with grape spirits, resulting in a range of styles from dry to sweet.

The other options do not accurately represent characteristics common to fortified wines. They may involve different aspects of wine production or service. For instance, fortified wines are not exclusively made from white grapes, as many varieties, including red grapes, can be used. Additionally, secondary fermentation is not a defining trait of all fortified wines; it is more commonly associated with sparkling wines like champagne. Finally, while some fortified wines can be enjoyed warm (like certain styles of port), they are not typically served in this manner across the board, with many being enjoyed chilled or at room temperature.