(C) Written letters
Written letters are a form of verbal communication (written communication), whereas gestures, facial expressions, and posture are all examples of non-verbal communication.
Source: Session 2: Verbal Communication, Table 1.2; Session 3: Non-verbal Communication, Table 1.5 — Chapter 1
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Examiners expect students to know the distinction: verbal communication uses words — spoken or written (letters, emails, reports, SMS). Non-verbal communication uses body language — gestures, facial expressions, and posture. Options A, B, and D are explicitly listed as non-verbal in Table 1.4/1.5 of the textbook. Option C (written letters) is listed under "Written Communication" in Table 1.2, which is a type of verbal communication.