Which instrument is used to measure the potential difference across components? A) Ammeter B) Thermometer C) Voltmeter D) Electroscope

What the question is really asking

You are being asked which instrument measures potential difference, which is another name for voltage, across an electrical component.

Matching each instrument to what it measures

• Ammeter: measures electric current (in amperes, $A$).
• Thermometer: measures temperature (in $^{\circ}C$ or $K$).
• Voltmeter: measures potential difference (in volts, $V$).
• Electroscope: detects the presence of electric charge, not voltage across a circuit component.

Why the voltmeter is correct

Potential difference is defined as energy transferred per unit charge: $$V = \frac{W}{Q}$$ A voltmeter is built to measure this voltage between two points, so it is placed in parallel across the component.

Quick check to avoid confusion

If the question asked for the instrument used to measure current through a component, the correct choice would be an ammeter, connected in series.