Which ingredient enhances the sweetness in sour cocktails?

The correct choice is sugar or simple syrup, as these ingredients are specifically designed to increase the sweetness in cocktails, particularly in sour cocktail recipes. Sour cocktails typically balance acidity from ingredients like citrus juice with sweetness to create a harmonious flavor profile. By adding sugar or simple syrup, bartenders can effectively counteract the tartness, producing a drink that is well-rounded and enjoyable to the palate.

Bitters are used to add complexity and depth to cocktails, often introducing herbal or spice notes, but they do not contribute sweetness. Fruit garnishes may provide fresh notes or visual appeal but do not have a significant impact on the overall sweetness of the drink. Sweet vermouth can add a degree of sweetness, but it is also a fortified wine with complex flavors, and it is typically used in specific cocktails rather than as a general sweetening agent in sour drinks.

Thus, sugar or simple syrup is the primary ingredient that directly enhances sweetness in sour cocktails, making it the best choice for this question.