Final answer:
To build a weight area in a graduated form, the appropriate design line to use is a curved line, as it creates a soft transition that increases in volume and achieves the desired graduated effect.
Step-by-step explanation:
The question refers to the use of design lines in creating a graduated form, specifically in a hairstyle or possibly in sculpture or other forms of visual art. In building a weight area in a graduated form, you would use a curved line. This is because a curved line creates a soft and progressive transition that increases in weight or volume, thus achieving the graduated effect desired. Curves provide a gradient or tapered approach to shaping the form, which is essential in graduating the form from less to more substantial, or vice versa, in an aesthetically pleasing way.