The answer is density. Milady names density specifically as an important design element when discussing balance in a haircut — balance being how evenly weight and shape are distributed across the finished cut.
Color, porosity, and texture (the other choices on this question) are all real haircutting/hair-science considerations, but they aren't the design element the textbook ties to balance specifically — density, meaning how many hair strands grow per square inch, directly affects how much visual weight a section of hair carries.
Milady Standard Barbering, 6th ed., Ch. 14, p. 396: "Hair density is an important design element when discussing balance."
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