Which of the following base shapes are used within a circle?

The correct base shape used within a circle is a triangle. In hairstyling, particularly when discussing the foundation of haircutting, a triangle shape is effective for creating volume and layers, as it allows for a gradual build-up of height when cutting. The triangle shape radiates from a point and expands outward, which can enhance texture and movement in the hair.

While other shapes, like squares and rectangles, are useful in various contexts, they do not share the same properties of softening edges and layering that a triangle does. A hexagon, while it can fit within a circle, does not align with the principles typically used in hairstyling base shapes. Triangles are fundamental for establishing form and are instrumental in achieving a desired look, making them vital in the hairstyling process.