Final answer:
A. All-purpose flour.
Most piecrust recipes call for all-purpose flour due to its versatility and moderate protein content, which gives a balance between flakiness and structure.
Step-by-step explanation:
Most piecrust recipes specify the use of A. All-purpose flour because it has a moderate protein content that makes it versatile for a wide range of baked goods, including pie crusts. All-purpose flour offers the right balance of flakiness and structure without being too tough, which can happen if a flour with a high protein content, like bread flour, is used. Pastry flour can also be used for pie crusts as it has a lower protein content, which makes very tender baked goods, but the standard choice in most recipes is all-purpose flour.