Which of the following instruments typically operates on the principle of thermal conductivity?

The correct choice, a thermistor, operates on the principle of thermal conductivity. Thermistors are temperature-sensitive resistors, and their resistance changes with temperature. This change in resistance is directly related to the thermal conductivity of the material and the surrounding environment. Since thermistors are designed to sense temperature based on the heat transfer characteristics of the materials they are made from, they utilize the principle of thermal conductivity effectively in their operation.

In contrast, pressure transducers measure pressure changes and convert the mechanical force into an electrical signal, which is not based on thermal conductivity. Thermocouples, while related to temperature measurement, operate based on the thermoelectric effect rather than thermal conductivity. Vortex flowmeters measure flow rates by detecting vortices shed by an obstacle in the fluid flow and are unrelated to thermal conductivity principles. Thus, the thermistor remains the only option that embodies the concept of thermal conductivity in its operational principle.