Final answer:
When cutting a triangular one-length below the shoulders haircut, the subsection taken is a blunt cut, which creates a clean, even line at the ends of the hair.
Step-by-step explanation:
When cutting a triangular one-length haircut that falls below the shoulders, the type of subsection taken is 3) Blunt cut.
A blunt cut is a technique used to create a clean, sharp line at the ends of the hair. It is characterized by cutting the hair straight across at the same length. This technique is especially popular for achieving a one-length look, as it ensures all the hairs are the same length, resulting in a sleek and even appearance.
Layers, graduation, and texturizing are techniques used to create different effects. Layers remove weight and add movement, graduation builds up weight, and texturizing is used to thin out the hair or add visual interest through varied lengths within the haircuts. However, these techniques do not apply to creating a uniform length like the triangular one-length haircut.