P — Ammeter (measures current in the circuit, connected in series)
Q — Nichrome wire / Resistor (the conducting wire whose resistance is being studied)
R — Plug key / Switch (used to open or close the circuit)
S — Cell / Battery (source of electric potential difference)
Source: Chapter 11 – Electricity, Section 11.5 (Activity 11.3, Fig. 11.5)
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Since the actual diagram is not visible, the answer is based on Fig. 11.5 of Activity 11.3, which shows a standard circuit for studying resistance. Examiners expect students to name all four components correctly — name only (or name + one-line function) is sufficient for 2 marks (½ mark each). Always match the component to its role: ammeter in series, key to control circuit, cell as source, and the wire/resistor as the component under test.