The question asks to identify a specific type of resistor whose electrical resistance changes in response to variations in temperature. Understanding the fundamental characteristics of different electronic components is crucial for correctly answering this question.

Thermistor Characteristics

A thermistor is a specialized type of resistor, whose name is derived from "thermal resistor." Its defining characteristic is that its electrical resistance is highly sensitive and changes predictably with changes in temperature. This unique property makes thermistors invaluable components in various applications for temperature sensing, measurement, control, and compensation.

• NTC Thermistors (Negative Temperature Coefficient): These are the most common type of thermistors. For NTC thermistors, their electrical resistance decreases significantly as the ambient temperature increases.
• PTC Thermistors (Positive Temperature Coefficient): For PTC thermistors, their electrical resistance increases as the ambient temperature increases. They are often used in current limiting applications or as resettable fuses.

Comparing with Other Electronic Components

It is important to understand why the other options provided do not fit the description of a component whose primary function involves resistance varying with temperature:

• Inductor: An inductor is a passive electronic component that stores energy in a magnetic field when electric current flows through it. Its main characteristic is inductance, and it opposes changes in current. While all materials have some temperature dependence, an inductor's primary purpose is not to have its resistance vary significantly with temperature for sensing.
• Geostat: This term is likely a misspelling of "rheostat." A rheostat is a type of variable resistor primarily used to control current by manually or mechanically adjusting its resistance. Its resistance change is typically not due to temperature variations but rather through physical adjustment by a user or system.
• Capacitor: A capacitor is another passive electronic component that stores electrical energy in an electric field. Its main characteristic is capacitance, and it opposes changes in voltage. Its ideal resistance is infinite (for DC), and it is not designed to have its resistance vary significantly with temperature for sensing purposes.

Conclusion on Thermistors

Based on their fundamental design and operational principle, thermistors are the only components among the given options specifically designed to exhibit a significant and predictable change in electrical resistance in response to temperature variations. This makes them perfectly suited for roles where temperature needs to be monitored or controlled, such as in thermostats, digital thermometers, and engine management systems.