The answer is fine. Milady states directly that fine hair has the smallest diameter and is generally more fragile, easier to process, and more susceptible to damage from chemical services than coarse or medium hair.
The book also notes some fine or very fine hair doesn't possess a medulla at all, part of what accounts for its smaller strand diameter. Coarse hair sits at the other end of the spectrum — the book separately describes it as more resistant to chemical processing, needing more time and product to achieve the same result.
Milady Standard Barbering, 6th ed., Ch. 10 (Properties and Disorders of the Hair and Scalp), p. 266: "Fine hair has the smallest diameter and is generally more fragile, easier to process, and more susceptible to damage from chemical services than coarse or medium hair."
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