Which of the following Belgian beer styles is most likely to exhibit assertively sour, acidic flavors?

Flanders Red Ale is renowned for its assertively sour and acidic flavors, making it the most suitable choice among the given options. This style typically undergoes a specific fermentation process that involves both brewer's yeast and wild yeast (often Brettanomyces), alongside lactic acid bacteria. These microorganisms contribute to the complex sourness that is characteristic of Flanders Red Ale, resulting in flavors that can range from tart cherry and fig to vinegar-like acidity.

In contrast, the other styles listed are not known for sourness. Dubbel, Tripel, and Witbier primarily feature malt-forward characteristics, with Dubbel showcasing rich caramel and dark fruit notes, Tripel focusing on light, fruity, and spicy flavors, and Witbier being known for its refreshing, citrusy, and herbal qualities stemming from the use of spices and wheat. While they may have some acidity, particularly from yeast characteristics, the level and intensity of sourness found in Flanders Red Ale are unique and significantly more pronounced.