Final answer:
The sensing portion of a bi-metallic stem thermometer is the coiled wire made of two different metals bonded together, which expands or contracts with temperature changes to provide a reading.
Step-by-step explanation:
The sensing portion of a bi-metallic stem thermometer is D) Coiled wire. Specifically, it refers to the two-layered strip formed by two different metals, such as brass and steel, that are bonded together. In a bi-metallic thermometer, this coiled wire acts as the temperature-sensitive element. When the temperature changes, the two metals expand or contract at different rates, causing the strip to curl or uncurl. This mechanical movement is then translated into a temperature reading on a calibrated scale. Bi-metallic stem thermometers are different from those that use a glass bulb with mercury or another liquid, which expands or contracts within the bulb and rises or falls in a tube to indicate temperature.