Which spice is commonly used in the brewing of Belgian witbiers?

Coriander is commonly used in the brewing of Belgian witbiers to impart a distinctive flavor that complements the beer's light and refreshing profile. This spice helps enhance the citrus notes often found in these wheat beers, balancing sweetness with a zesty, aromatic quality.

Witbiers are traditionally brewed with a blend of spices and citrus peels, and coriander is one of the classic components that contribute to the overall complexity of the beer. Its use harks back to historical brewing practices in Belgium, where diverse spices were commonly added to enhance flavor without the use of hops.

While ginger, cinnamon, and allspice are spices that can be used in various beer styles, they do not have the same traditional association with Belgian witbiers as coriander does. Thus, coriander stands out as the quintessential spice for this particular style, making it the correct answer.