The answer is coarse. Milady states coarse hair usually requires more processing than medium or fine hair and may also be more resistant to chemical processes.
That's the reverse of fine hair, which the same section calls more fragile and easier to process — and therefore more susceptible to chemical damage, not more resistant to it. Porosity is a related but separate property from texture; this specific question is about strand diameter (coarse/medium/fine), where coarse is the resistant end of the scale.
Milady Standard Barbering, 6th ed., Ch. 17 (Chemical Texture Services), p. 583: "Coarse hair usually requires more processing than medium or fine hair and may also be more resistant to chemical processes."
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