What is streusel?

Streusel is best defined as a sweet, crumbly topping made from a mixture of flour, sugar, and fat, often with the addition of spices, nuts, or oats. This mixture is typically crumbled over pies, muffins, or coffee cakes before baking, creating a delightful texture and flavor contrast to the dish underneath. The primary purpose of streusel is to add a crunchy, sweet overlay that enhances the overall taste and visual appeal of baked goods.

In this context, while there may be other methods of adding flavors and textures to pastries, choices like frosting, pastry dough, and cake batter do not capture the essence of streusel. Frosting refers to a sweet topping typically made from sugar and fat, used primarily on cakes. Pastry dough is a foundational element used to create pastries, while cake batter refers to the mixture that is baked to form a cake. Thus, the distinct nature and purpose of streusel as a crumbly topping clearly mark it as the correct answer.