The answer is personal referrals. Milady states this directly in its marketing section: personal referrals are the best evidence of pleased and satisfied clients.
The book notes that other marketing tools — like window displays and advertising — shouldn't be relied on exclusively for new business, even though they're still effective. The distinction it draws is that personal referrals are specifically evidence of client satisfaction, not just a promotional channel like the other three choices.
Milady Standard Barbering, 6th ed., Ch. 15 (Men's Hair Replacement), p. 496: "Personal referrals are the best evidence of pleased and satisfied clients."
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