The answer is wiry. Wiry hair has a hard, glassy surface that makes it tougher to bend or curl than regular hair.
Wiry is a finish, not a diameter measurement — you can have fine wiry hair or coarse wiry hair, since texture (fine, medium, coarse) and this glassy quality are two separate things the exam tests separately. The glass-like look comes from how tightly the cuticle lies against the shaft, not from how thick the strand is.
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