Which of the following is a type of fortified wine?

Sherry is indeed a type of fortified wine. Fortified wines are those to which additional alcohol has been added, typically in the form of grape spirit. This process not only increases the alcohol content but can also influence the flavor and aging potential of the wine. Sherry, which originates from the Jerez region of Spain, undergoes fortification after fermentation, leading to diverse styles ranging from dry to sweet.

Other wines listed in the options represent different categories. Merlot is a type of red wine grape used to produce various red wines but is not fortified. Cava is a sparkling wine from Spain, and while it may undergo secondary fermentation to create its fizz, it is also not fortified. Riesling is a white grape variety known for producing both dry and sweet wines but, like Merlot and Cava, it does not fall under the category of fortified wines.