The answer is genetics. It's a counterintuitive-sounding answer at first — "acquired" makes it sound environmental — but Milady is direct about attributing acquired canities (the graying/whitening of hair that develops with age) to genetics.
The other choices on this question (previous over-processing, a fungus, poor nutrition) describe real hair conditions with real causes, but none of them is what the textbook names as the cause of age-related canities specifically. "Acquired" here just distinguishes it from congenital canities (present from birth) — it doesn't mean the cause is external.
Milady Standard Barbering, 6th ed., Ch. 10, p. 260: "Acquired canities develops with age and is the result of genetics."
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More real Disorders questions with sourced answers: Where on the Body Does Tinea Barbae Occur.
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