Option analysis:
(A) In a boiler fusible safety plug, the plug casing/body is made of gunmetal or bronze, while the inner core consists of a low-melting point alloy such as pure tin (melting point $\approx 232^\circ\text{C}$) or lead-tin alloy.

A Resistance Temperature Detector (RTD or Resistance Thermometer) measures temperature based on the predictable change in electrical resistance of a pure metal wire (such as platinum, Pt100) with temperature. Because the fractional resistance changes are relatively small, a Wheatstone bridge circuit (in 2-wire, 3-wire, or 4-wire configuration) is used to accurately measure the resistance variations.

Option analysis:

• Pyrometer: Optical/radiation sensor measuring emitted radiant energy without electrical bridges.
• Bimetallic thermometer: Mechanical sensor based on differential thermal expansion of two bonded metals.
• Thermocouple: Operates on Seebeck effect measuring millivolt potential differences, not Wheatstone bridges.
• Resistance thermometer: Correct, operates fundamentally using a Wheatstone bridge circuit to measure resistance.