The answer is alcohol. Milady's phrasing lines up closely with the question: antiseptics "generally contain a high volume of alcohol." Antiseptics are meant for use directly on skin (unlike disinfectants, which are only for implements and surfaces), and alcohol's high concentration is what gives them their germ-killing effect at skin-safe strength.
Ammonium, formaldehyde, and sodium hypochlorite (bleach) — the other choices — are all disinfectant or chemical-processing ingredients, not the defining ingredient of antiseptics; several of them (formaldehyde, sodium hypochlorite) would be genuinely unsafe to apply directly to skin the way antiseptics are designed to be used.
Milady Standard Barbering, 6th ed., Ch. 4 (Infection Control), p. 79: "Antiseptics... generally contain a high volume of alcohol."
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