Match instruments to the principal quantity they measure: (A) Hot-wire instrument, (B) PMMC instrument, (C) Electrostatic voltmeter, (D) Lock-in amplifier.

Introduction / Context:
Measurement systems often hinge on choosing an instrument that matches the signal characteristic: RMS value, DC-only measurement, very high impedance voltage sensing, or phase-sensitive detection. This mapping reinforces key associations used in labs and exams.

Given Data / Assumptions:

• Hot-wire instruments respond to heating, hence read true RMS.
• PMMC (Permanent Magnet Moving Coil) instruments respond to DC values.
• Electrostatic voltmeters sense electrostatic force and measure voltage only.
• Lock-in amplifiers use phase-sensitive detection to measure signals buried in noise and report amplitude/phase relative to a reference.

Concept / Approach:
Use the defining physical principle of each instrument to match to the measured quantity.

Step-by-Step Solution:

Verification / Alternative check:
In AC measurement, hot-wire meters read correctly regardless of waveform due to thermal response; PMMC requires rectifiers for AC and then indicates average/DC equivalent; electrostatic devices are preferred for high-voltage, high-impedance measurements; lock-ins extract narrowband components at the reference frequency and phase.

Why Other Options Are Wrong:

• A-1, B-4, C-2, D-3: Hot-wire does not measure voltage-only; lock-in is not strictly RMS meter.
• A-3, B-2, C-1, D-4: Misassigns PMMC and lock-in functions.
• A-2, B-3, C-4, D-1: Interchanges multiple roles.

Common Pitfalls:

• Assuming PMMC reads AC without rectification.
• Confusing RMS indication with average-responding meters.

Final Answer:
A-3, B-4, C-1, D-2