The answer is metal. Milady's electrical-safety rule is direct: "Do not touch any metal while using an electrical appliance." Metal conducts electricity, so touching a metal surface — a chair frame, a pipe, a faucet — while operating a live clipper or other appliance creates a real shock risk if the appliance has any fault.
Plastic, rubber, and glass — the other choices — are all poor electrical conductors and don't carry the same risk; this is exactly why appliance housings and cords are typically insulated with plastic or rubber rather than left bare. This pairs directly with the "disconnect by the plug, not the cord" question above — both are basic shop electrical-safety facts.
Milady Standard Barbering, 6th ed., Ch. 8 (Basics of Electricity), p. 210: "Do not touch any metal while using an electrical appliance."
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