The answer is drug or tobacco use. Milady states plainly: questions regarding drug or tobacco use are permitted, and an employer may even require agreement to drug-testing policies as a condition of employment.
The other three choices — medical conditions, date of birth, and disabilities — sit in the same list of interview topics the book flags as legally restricted or off-limits in most contexts. Drug and tobacco use is the specific exception the book calls out as fair game for an employer to ask about directly.
Milady Standard Barbering, 6th ed., Ch. 19 (Preparing for Licensure and Employment), p. 729: "Drug use or smoking. Questions regarding drug or tobacco use are permitted. In fact, the employer may obtain the applicant's agreement to be bound by the employer's drug and smoking policies and to submit to drug testing."
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