The answer is hydroxide relaxers. Milady's own description is a near-exact match to the question: hydroxide relaxers are strong alkalis capable of swelling the hair shaft up to twice its normal diameter during processing.
That swelling is precisely why hydroxide relaxers require careful timing and strand testing — over-processing at that level of swelling is what causes breakage. The other answer choices (neutralizers, waving solutions, thio relaxers) describe genuinely different products with different chemistry.
Milady Standard Barbering, 6th ed., Ch. 17 (Chemical Texture Services): "Strong alkalis that can swell the hair up to twice its normal diameter are hydroxide relaxers."
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